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The sense of smell informs an organism about the chemical com-position of the external environment1. The olfactory sensoryepithelium is composed of a few million olfactory sensory neu-rons (OSNs), which are generated in situ from stem cells. Like inother epithelia, cell renewal persists throughout adult life toreplace OSNs, which have a lifespan of weeks to months. Amature OSN is a bipolar neuron with a single, short dendriteending in cilia that protrude into the mucus covering the epithe-lium, and a single, unbranched axon coursing from the nasal to the cranial cavity (Fig. 1a). In the olfactory bulb, the axon of an OSN terminates within a glomerulus, where it makes synap-tic contacts with the dendrites of second-order neurons andinterneurons. Glomeruli are globose neural structures of ~100 µm in diameter, numbering ~1800 in the bulb of an adultmouse (Fig. 1b). Olfactory glomeruli have been viewed as func-tional units since they were first described in the 19th century,but it was not certain what type of unit a glomerulus representsuntil the discovery of odorant receptor genes.

One in thirty genes is for smellStimulation of odorant receptors by odorous molecules triggersthe sensation of smell. Mammalian odorant receptor (OR) geneswere isolated in 1991 (ref. 2). These genes represent the largestmammalian gene families, composing a staggering ~1000 of the≥30,000 genes in mouse and human (for reviews, see refs. 3–5).OR genes encode proteins with a putative seven-transmembrane-domain structure (Fig. 2a), a hallmark of G-protein coupledreceptors. Mammalian OR genes have no introns in their codingregion, an unusual property that facilitates their identificationfrom genomic sequence. A first draft of the human OR reper-toire has recently been reported6,7. Of the ~1000 human ORsequences, only one-third have a full-length, uninterrupted openreading frame; the remaining two-thirds are pseudogenes. In con-trast, most OR sequences in mouse are intact and can encodefunctional receptors8. The higher integrity of the mouse ORrepertoire may be related to the greater dependence of the mouseon its sense of smell for survival.

The genomic distribution of the human and mouse OR reper-toires does not follow any obvious logic: the genes are haphaz-

How smell develops

Peter Mombaerts

The Rockefeller University, 1230 York Avenue, New York, New York 10021, USA

Correspondence should be addressed to P.M. (peter@rockefeller.edu)

Published online: 29 October 2001, DOI: 10.1038/nn751

The mouse’s sense of smell is built of ~1000 input channels. Each of these consists of a population ofolfactory sensory neurons that express the same odorant receptor gene and project their axons tothe same targets (glomeruli) in the olfactory bulb. A neuron must choose to express a singularreceptor gene from a repertoire of ~1000 genes, and its axon must be wired to the correspondingglomerulus, from an array of ~1800 glomeruli. Genetic experiments have shown that the expressedodorant receptor specifies axonal choice of the innervated glomerulus, but it is not the only determi-nant. The mechanisms of odorant receptor gene choice and axonal wiring are central to thefunctional organization of the mammalian olfactory system. Although principles have emerged, ourunderstanding of these processes is still limited.

ardly spread over dozens of sites in the genome, as singletons orin clusters (Fig. 2c). OR genes invade or enclose unrelated genefamilies, such as the T-cell receptor locus9 and the beta-globinlocus10, respectively. Within a cluster there is no orderly arrange-ment of the OR genes; transcriptional orientation can be eitherway, and intergenic distances are variable. In sum, the genomicexploration of mammalian OR repertoires has provided few cluesabout OR gene regulation.

A mosaic in the noseHow is the expression of 1000 OR genes distributed over the mil-lions of OSNs in the nose? Single-cell experiments with RT-PCR(reverse transcriptase-polymerase chain reaction) indicate thata singular OR is expressed in a given OSN11. (An exception is aspecific pair of ORs that are coexpressed in some rat OSNs12.)Moreover, an OR gene is expressed from a single allele in a givencell13, such that within an organism, OSNs expressing either thematernal or the paternal allele coexist mosaically and in approx-imately equal numbers14,15. A rationale for the peculiar featureof monoallelic expression is that it ensures expression of a sin-gular receptor type per OSN, in a situation of extensive allelicOR polymorphisms. A particular OR gene is expressed in OSNsthat are interspersed with OSNs expressing other OR genes16,17.The olfactory epithelium is thus a complex mosaic of distinctOSN populations. Scattering OSNs with the same odorant sen-sitivity over a wide area may increase the likelihood that odor-ous molecules, inhaled with whiffs of air, will interact with thecognate receptors along their turbulent trajectory in the nasalcavity.

The epithelium is subdivided into four zones of OR geneexpression (Fig. 3a). Each occupies ~25% of the surface of theepithelium16,17 and comprises the cell bodies of a vast majorityof OSNs expressing a particular OR gene (Fig. 3a). The punctatepattern of expression is often said to be ‘stochastic,’ in the absenceof an obvious order, but statistical proof for randomness muststill be provided. Limited evidence exists for a subtle, non-sto-chastic pattern14,18.

The zonal organization of the epithelium was not predictedby anatomical and physiological studies, but resulted from the

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molecular characterization of OR gene expression by in situhybridization. The epithelial zones do not correlate with obvi-ous regional preferences in odorant sensitivity within the epithe-lium, with obvious differences in developmental history, or withthe genomic distribution of OR genes. The zones are an innatefeature of the epithelium: in fetal development, OR genes areturned on synchronously and directly within the appropriatezone19,20. This also occurs in mutant mice that lack olfactorybulbs19, indicating that the epithelial pattern is established inde-pendently from target-derived influences. As the epithelial zonesproject roughly onto four domains in the bulb21, the zones mayrepresent a compartmentalization of the olfactory system intofour subdivisions of ~250 functional types (forming a total of~1000 OSN populations), reducing the complexity of the wiringproblem (Fig. 3a). Axon pathfinding may thus be regarded as atwo-step process, with a coarse mechanism to establish the correctzone-to-domain map, and a fine mechanism to innervate a spe-cific glomerulus out of ~250 choices. Suggestive of this hypoth-esis is the zone-specific expression of the olfactory cell adhesionmolecule (OCAM), an NCAM homolog that is a candidate axonguidance molecule22,23. Two regulators of G-protein signaling(RGS genes) are expressed differentially within mature OSNs ofdistinct zones, suggesting that zone-specific factors may modulateolfactory signal transduction24.

A thousand points of lightWhereas the epithelial expression pattern of a given OR is dom-inated by the punctate, seemingly random distribution withinthe confinements of a zone, the situation in the bulb is dramati-cally different. Axons of OSNs that express the same OR convergewith extraordinary precision onto a few specific glomeruli amongthe ~1800 possible glomeruli. The first evidence for this was pro-vided by in situ hybridization experiments that relied on traceamounts of OR mRNA within the axon terminals25,26. Ultrasen-

sitive hybridization techniques with specific OR probes revealedbilaterally symmetrical spots at similar positions, interpreted toreflect the convergence onto glomeruli of axons from some OSNsthat express the same OR. Evidence that this is the case for allaxons of the same OR type came from gene-targeting experi-ments, in which expression of a particular OR, the P2 gene, wascoupled to that of the axonal marker taulacZ27. Coexpression wasachieved by inclusion of a viral internal ribosome entry site(IRES) that renders OR transcripts bicistronic, resulting in trans-lation of both OR and marker. In these P2-tagged mice, labeledaxons converge to a few glomeruli in the bulb that are located insimilar positions from bulb to bulb and from mouse to mouse,although the positions are not stereotyped in a strict sense; theyshow local variability. These genetic tracing experiments revealeda symmetry within the bulb, which was suggested earlier by in situ hybridization data25: P2 glomeruli are located invariablyboth in the medial and lateral halves of each bulb, and the planeof symmetry is such that the lateral glomeruli are more dorsaland more anterior. The bulb is a duplicate of two half-bulbs28,29,each displaying a ‘thousand points of light,’ ~1000 glomeruli thateach correspond to a specific OR gene.

A century after the anatomical description of glomeruli asiterated units, molecular biology has finally provided an answerto what a glomerulus represents: it receives axonal input fromOSNs that express a particular OR gene, thus providing for inte-gration of information and an increase in the signal-to-noiseratio. The concept of glomerular convergence is consistent witha widely accepted view of olfactory coding, in which the particularcombination of activated glomeruli informs the brain what thenose is smelling21,30.

This topographic organization of axonal projections poses aphenomenal wiring problem: axons of ~1000 populations ofOSNs, each expressing a different OR gene, must be sorted out, induplicate, onto ~1000 glomerular targets31.

Keep it shortTwo basic and related issues in the development of the mam-malian olfactory system are the mechanisms of OR gene choiceand of glomerular convergence. How does an OSN choose toexpress one of 2000 OR alleles, and how does its axon find itsway to the glomerulus appropriate for that OR?

Transgenesis with tagged mouse OR genes has been appliedto define the minimal genomic sequences that can mimic ORgene expression patterns. The objective is to extract an OR genefrom its cluster context, and to determine if it functions

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nature neuroscience supplement • volume 4 • november 2001 1193

Olfactoryepithelium

Vomeronasalorgan

Olfactorybulb

1800 glomeruliin the mainolfactory bulb

Accessoryolfactory bulb

Fig. 1. Anatomy of the olfactory system in mice. (a) Sagittal sectionthrough the head of a mouse of the OMP-taulacZ strain27. Medial view onthe right half-head in wholemount. Mature OSNs express the olfactorymarker protein (OMP), whose function is unknown. In this strain,mature OSNs also express β-galactosidase, which converts the color-less chemical X-gal into a blue precipitate. Fusion to tau enhances deco-ration of axons and axon terminals. The olfactory epithelium covers theseptum and contains the dendrites, cell bodies and proximal part ofaxons of OSNs. The axons terminate within glomeruli in the olfactorybulb. The vomeronasal organ is thought to be specialized in the detec-tion of pheromones, but it does not have monopoly on this function. ORgenes are typically not expressed in the vomeronasal organ. The trian-gular structure between the vomeronasal organ and the main olfactoryepithelium is the septal organ, which has a poorly defined function. (b) Dorsal view of the olfactory bulb of a mouse of the OMP-taulacZstrain. An OSN axon terminates within a single glomerulus. Axons ema-nating from the vomeronasal organ terminate in the accessory olfactorybulb, a specialized region.

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autonomously when transplanted elsewhere in the genome. Thisis a standard approach to tissue-specific gene control, but in 10years only 3 transgenic studies have been reported, and no coher-ent view has emerged. In a first study32, a 6.7 kb segmentupstream of the translation start site of the OR known as M4 hadthe capacity to convey a punctate and zonally-appropriate pat-tern. Although only a single transgenic line exhibited expressionin the correct zone, this study pioneered the notion that OR genechoice may be under short-range regulation. A second groupreported33,34 that a yeast artificial chromosome (YAC) of 300 kbcontaining the tagged M12 OR is sufficient for expression, but itled to grossly abnormal patterns of expression in the epitheliumand of axonal convergence in the bulb. In a third study, a YAC of200 kb was sufficient for expression of the MOR28 OR, but trun-cated YACs of 90 and 180 kb were not expressed15,35,36. OSNsexpressed either the transgenic MOR28 or the endogenousMOR28 but not both. This phenomenon, termed ‘mutuallyexclusive expression,’ was observed in a more rudimentary formin the first study32. A striking manifestation of this phenomenonis that coinjection of differentially tagged YAC transgenes doesnot result in coexpression of the tagged OR genes in the samecells, although they are cointegrated in the same locus and areidentical except for the genetic tags35. An unexplained problemwith the MOR28 YAC transgenics, however, is that axons of trans-gene-expressing OSNs do not project to the endogenous MOR28glomeruli: every transgenic line has its own, distinct glomeru-lus, which can be remarkably close to the native glomerulus with-out cross-innervation.

Thus, at one extreme, forone OR (M4), a few kilobasesof genomic DNA are suffi-cient for expression in a smallsubset of OSNs within azone, whereas another OR(MOR28) requires 200 kb ofgenomic DNA to accomplish

this. These findings need not be contradictory, because noextremely short MOR28 transgenes were tried. It also should berealized that the MOR28 OR gene9 happens to be embeddedwithin the T-cell receptor locus, which undergoes a series of DNArearrangements during T-lymphocyte development. The MOR28YACs may thus contain regulatory information for two distinct,complex gene families, such that truncations may expose the ORtransgene inadvertently to silencing effects from elements con-trolling the T-cell receptor locus. An alternative explanation isthat regulatory regions can be located at widely varying distancesdepending on the OR.

The property of OR-like choice can be recruited by a minipro-moter of a gene that is specifically expressed in all mature OSNs.The transgene landed near an OR gene and partially displayedits expression pattern37, suggesting that the regulatory machineryfor OR gene expression can exert influence on olfactory-specificpromoters at a longer distance.

No business as usualHow do we actually determine that the ‘correct’ pattern of ORgene expression has been reproduced in transgenic mice? Theminimal requirement of OSN-specific expression is not suffi-cient. The problem is fundamentally different from the regula-tion of most other tissue-specific genes, for which transgenicsuccess can be evaluated in terms of spatial and temporal coex-pression with the endogenous counterpart. Mutually exclusiveexpression means that the OR transgene is not coexpressed inOSNs that express the endogenous OR gene35. It seems that the

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Secondmessenger

Activeadenylcyclase

Odorant

Odorantreceptor

ActiveNa+/Ca2+

channel

GTP

cAMP

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Fig. 2. OR genes. (a) Seven-transmembrane domain struc-ture of an OR. The extra- andintracellular ends are relativelyshort. (b) Olfactory signal trans-duction pathway in higher verte-brates. The odorant interactswith the OR, resulting in G-pro-tein activation, stimulation ofadenyl cyclase activity, openingof a cyclic-nucleotide gatedchannel by elevated cAMP levels,influx of cations and depolariza-tion. (c) Distribution of ORgenes in the human genome. Allhuman chromosomes exceptchromosomes 20, 22 and the Y-chromosome harbor OR genes.They are typically clustered, butsome genes form singletons. Thelength of each horizontal bar isproportional to the number ofOR sequences (genes pluspseudogenes), with the smallestbars corresponding to single-tons. Chromosome 11 has 42%of all human OR sequences.Drawn after ref. 6.

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apparatus for OR gene choice scans the repertoire (two alleles of~1000 OR genes, plus the OR transgenes) and chooses a singlemember for expression. Thus, singular, monoallelic and mutu-ally exclusive expression (Fig. 4) may be manifestations of thesame regulatory process.

Glomerular co-convergence of axons from transgene-express-ing OSN with axons of OSNs expressing the endogenous OR geneshould be a decisive proof of transgenic success. The argumentis that the identity of a glomerulus is intimately intertwined withthe identity of the OR expressed by OSNs whose axons innervatethat glomerulus. Deviations from the endogenous glomerulartarget are a sensitive indicator that the transgene has failed tomimic the expression pattern; for instance, the correct level ofOR expression may not have been achieved. According to thisstringent requirement, the expression pattern of any OR gene hasnot been replicated in transgenic mice, although one study camevery close35.

Switching genes?Another means to determine the mechanisms of OR choice is tosearch for common sequence elements in OR genes, a standardstrategy for most genes with tissue-specific regulation. Inspec-tion and comparison of sequences upstream of the putative tran-scription start site of OR genes have not revealed convincingfeatures with sequence conservation. The overwhelming impres-sion is the lack of consistent promoter motifs38–41. Regulatorymotifs may be so small, degenerate or scattered as to evade detec-tion with available algorithms.

Singular, monoallelic and mutually exclusive expression, andthe absence of obvious promoter motifs smell of DNA

rearrangements as a mechanism that an individual OSN usesto pick its OR. These DNA changes would switch on the expres-sion of one allele of a single OR gene. A parallel can be drawnwith variant surface glycoprotein genes in Trypanosoma cruzi;one member of a family of 1000 genes is chosen for expressionby gene conversion into an expression site42. The analogy withDNA rearrangements of immunoglobulin and T-cell receptorgenes is less appropriate, as it is now virtually ruled out that thecoding regions of OR genes are assembled from gene segments.The cassette model, which is still speculative, can be tested byexamining genomic DNA derived from an OSN that expressesa defined OR.

The OR as a guidance moleculeGene-targeting experiments altering the P2 coding region con-sistently affect glomerular convergence, implying the OR itselfin the mechanisms of axon guidance27,43. Deletion of the OR cod-ing region destroys glomerular convergence; instead, the labeledaxons project diffusely over a large region of the surface of theolfactory bulb43. A caveat of this experiment, which has not yetbeen resolved, is that other OR gene(s) may be expressed in thelabeled neurons, which would project their axons to the glomeruliappropriate for the expressed OR(s) but independently of theinvolvement of the OR in this process. More insightful conclu-sions came from OR swap experiments in which the P2 codingregion is replaced by another related or unrelated OR gene27,43. Ineach of the four reported OR swaps, axons converge to glomeru-lar targets distinct from either the P2 glomeruli or the glomerulifor the ‘donor’ OR. Thus, the ‘address’ in the bulb to which theaxons navigate is determined by the specificity of the expressed

OR, but the OR is not the only determinant. Anintriguing observation is that axons ofvomeronasal sensory neurons that express an ORfrom a targeted locus converge to glomeruli inthe accessory olfactory bulb44, although theputative chemosensory receptors normallyexpressed in those neurons30 have no significantsequence homology to ORs. How unrelatedseven-transmembrane domain proteins canimpart axon guidance information remainsentirely elusive.

The OR is clearly not the only determinant,as in no reported case does expression of a dif-ferent OR rerout axons to the cognate glomeruli.What could the other determinants be? A variety

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Fig. 3. Patterns of connectivity between olfactoryepithelium and olfactory bulb. (a) The olfactoryepithelium of mouse and rat can be subdivided in fourzones of equal surface area. A given OR gene isexpressed in OSNs whose cell bodies are restrictedto a zone. Their axons converge onto one or a fewglomeruli in each of the two half-bulbs. Shown here isthe medial face of the right bulb; AOB is accessoryolfactory bulb. Epithelial zones correspond to equiva-lent domains in the bulb, although the precise bound-aries of the bulbar domains remain to be defined.Drawn after ref. 28. (b) Glomeruli for a given OR donot occupy stereotyped positions in the bulb, butexhibit local permutations. Their position has adegree of uncertainty, both in absolute and relativeterms. Shown are glomeruli for three different ORs,occupying a variety of positions within an area of ~30glomeruli. Drawn after ref. 14.Bo

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of molecules are expressed in axonal populations that converge tosome but not other glomeruli, notably OCAM (olfactory cell adhe-sion molecule)22,23 and neuropilin-1 (refs. 29, 45, 46). The globalcontribution of semaphorin3A (ref. 46) and NCAM (neural celladhesion molecule)47 to axon pathfinding has been tested inknockout mice, whose axonal pathways and glomeruli are broad-ly altered.

A more focused approach has been to assess the effect of amolecular deficiency on the convergence of axons from P2-expressing OSNs, because P2-tagged mice permit the most con-venient and precise assessment of glomerular position. Crossingthe P2-tagged mice with various types of knockout mice yield-ed essentially negative results. The P2 glomeruli turn out to beindependent of all the following: an intact olfactory signal trans-duction pathway (mutations in Golf (a G protein subunit) anda subunit of the cyclic nucleotide-gated channel)48–50, second-order neurons in the olfactory pathway (Tbr-1 mutants, pre-cluding the development of mature mitral/tufted cells)51,interneurons in the bulb (Dlx-1/2 mutants, resulting in theabsence of periglomerular cells)51 and neutrophins and neu-rotrophin receptors (BDNF (brain-derived neurotrophic factor),NGF (nerve growth factor), TrkA, TrkB and TrkC mutants)52.However, the P2 glomeruli need not be representative of the olfac-tory system at large. A functional heterogeneity within the olfac-tory system was revealed by crossing mice lacking a cyclicnucleotide gated-channel subunit to mice with a tagged OR geneknown as M72: axons converge to multiple, smaller glomerulithat are located over a broader area50. Thus, the complexity ofthe wiring problem cannot be reduced to examining one of the1000 circuits, the population of P2-expressing OSNs.

Axons of P2-expressing OSNs terminate in a small area of thesurface of the bulb during fetal development, and the process ofglomerulization extends over a period of several days53. M72-expressing OSNs, however, project initially to a broad area, andthen coalesce abruptly into glomerular structures54—a furtherindication that the olfactory system does not consist of 1000channels that develop in identical ways. Other studies of globaldevelopment of the glomerular array emphasize that OSN axonsinterdigitate with the processes of radial glia55, or that OSN axonshave an autonomous propensity to coalesce during development

into glomerular-like struc-tures56. The radial glia are thusan interesting cell type in thebulb whose role in the forma-tion of specific glomerulideserves attention.

Activity-dependence of theolfactory systemIn other sensory systems, theformation of topographic pro-jections during development isgenerally thought to be dividedin an early phase of activity-independent processes followedby a refinement of the patternby activity-dependent process-es. With ‘activity’ is meant neu-ronal activity, typically electricalactivity, which is stimulus-evoked or spontaneous. Block-ing the olfactory signaltransduction pathway (Fig. 2b)

by knocking out a key subunit of the cyclic nucleotide-gatedchannel alters the olfactory sensory projections. This finding wasshown in two independent ways. First, whereas the P2 glomeruliappear to form normally in the absence of a functional trans-duction channel49,50, the M72 glomeruli do not50. Mosaic coex-istence in a mouse of M72-expressing OSNs that do or do notexpress the channel was engineered by using the phenomenonof monoallelic expression of OR genes50. The axonal projectionsof channel-positive (functional) and channel-negative (inoper-ative) OSNs sort out into separate glomeruli, suggesting that neu-ronal competition modulates the final map. Second, a similarmosaic situation was engineered, but globally in OSNs, relyingon mosaic X-inactivation in females and on the X-chromosomelocation of the channel subunit57. In young females, channel-positive and channel-negative neurons are present in similarnumbers, but with time the channel-positive neurons becomepredominant, presumably as a result of competition. Yet a smallfraction of glomeruli innervated by channel-negative neuronssurvives in older mice, and blocking odorant exposure of theolfactory epithelium by surgical closure of one nostril increasesthe number of such glomeruli. The interpretation is that differ-ential patterns of odor-evoked activity determine the glomeru-lar pattern.

These two manifestations of the activity-dependence of theolfactory system may explain why the glomerular array is not asterotyped arrangement of glomeruli, but shows slight variationsboth in the absolute and relative positions of specificglomeruli53,58. These ‘local permutations’14, which are best eval-uated by comparison of the two bulbs of the same individual,reflect positional uncertainty within ~2% of the surface area ofthe bulb14,58 (Fig. 3b). They probably underlie slight variationsseen in functional activity patterns obtained with intrinsic sig-nal imaging59.

Resurrecting the sense of smellThe olfactory system undergoes self-renewal throughout adultlife. OSNs have a limited lifespan, and when a new OSN is bornin an adult individual, its axon must extend along a long routeto the correct glomerulus. Because odor quality remains constantthroughout the life of an individual, the glomerular array must be

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No expression

Cells otherthan OSNs

OSNs

Expresses eORM Expresses eORP Expresses TgOR

Expresses other OR

Fig. 4. Singular, monoallelic and mutually exclusive expression. Shown are segments on three homologous pairsof chromosomes within nuclei. OR genes are represented as beads, arrayed in clusters. The integrated ORtransgene(s) (TgOR) is shown as a larger sphere on one chromosome. OR genes are specifically expressed inOSNs. For a given endogenous OR gene (eOR), either the maternal (M) or paternal (P) allele is expressed in aninvididual OSN. In an outbred population, the M and P alleles in a given individual may be different such that theypotentially encode ORs with distinct ligand specificities. The TgOR is expressed in a mutually exclusive fashionwith both the maternal and paternal eOR counterparts. Most OSNs express other OR genes. In each OSN, asingular OR type is expressed.

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constant to a certain degree. The P2 glomeruli have been the sub-ject of two different approaches examining the regeneration ofP2-expressing OSNs.

First, in P2-tagged mice of which all OSN axons, includingthose of P2-expressing OSNs, were surgically cut, the new cohortof tagged axons formed multiple glomeruli, suggesting that pre-cise guidance cues are no longer available in adult mice60. How-ever, surgical intervention produces massive lesions that maycause inadvertent damage and formation of scar tissue that devi-ates axons. A gentler type of lesioning such as destroying OSNs byirrigation of the epithelium with a detergent allows the patternof axonal projections to be largely restored, as was shown inanother strain of mice, with tagged OSNs61.

Second, an inducible genetic system has been developed toselectively kill P2-expressing OSNs, either throughout fetal andperinatal life, or transiently later in life62. Glomeruli form prop-erly after ablation of OSNs that express the manipulated P2 gene,yet these glomeruli are distinct from the native P2 glomeruli,before and after regeneration. As the cell killing reaches an effi-ciency of up to 98%, the newly formed axons seem to be able tofind their way back with as little as 2% of original population ofP2-expressing OSNs left.

A posteriori considerationsOR promoter regions are likely to be compact and proximal tothe transcription unit, an idea indicated by the genomic archi-tecture of the OR repertoire, which can be characterized as ‘hap-hazard’5–7. The seemingly non-orderly arrangement of OR genesthroughout the genome (Fig. 2c) may reflect the containment ofregulatory sequences within a small unit of DNA. It is possible,however, that because of this haphazard distribution, certain ORgenes are under short-range control, whereas others are depen-dent on long-range systems.

Another principle is that as OSNs that express the same ORgene project their axons to the same glomeruli, the OR itself couldbe deployed as a navigational tool, guiding axons as to where tocoalesce and form a glomerulus. If the OR were not involved atall, a separate and elaborate set of guidance factors would haveto coevolve to specify the sites of glomerular convergence, andthis seems unlikely.

What does the map mean?Five years after the first evidence for the role of ORs in axonalwiring27, we are still struggling to understand the mechanisms.If an intact olfactory transduction pathway is not required forthe formation of P2 glomeruli, how do the P2 molecules func-tion in axon guidance? Do they recognize perhaps each other?Which mutations in an OR are necessary or sufficient to formnovel glomeruli in the bulb? What makes axon terminals of likeOSNs coalesce into glomeruli? And what, if anything, do ORsrecognize in the bulb? Do they recognize a thousand distinctchemoattractive cues, or do they locate their spatial coordinatesby reading positional information encoded by a few gradients?

Another issue is whether the arrangement of glomeruli acrossthe surface of the bulb has an intrinsic functional meaning. Doesit matter for olfactory coding that P2 or M72 glomeruli are locat-ed where they are? Or is the glomerular array simply one of manypossible assortments, each of which would have given the mousean equally fine sense of smell? Is the map perhaps merely the con-sequence of the strategy of axonal convergence—thus ultimatelya developmental by-product, and a red herring when a deepermeaning is sought? These questions are likely to keep olfactoryresearchers following their noses for some time.

RECEIVED 17 JULY; ACCEPTED 19 SEPTEMBER 2001

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