The olfactory system provides a unique model for developmental neurobiology. Precise targeting of axonal projections from sensory neurons located in the olfactory epithelium to specific neurons in the olfactory bulb establishes a highly refined spatial sensory map. Distinctively, this process is not restricted to embryonic stages, but continues during the entire life of mammals. A number of secreted and membrane molecules have been implicated in guidance and targeting of olfactory sensory neurons. Here we describe olfactorin, the protein product of the mouse Umodl1 gene, as a potential new element in this process. Olfactorin is a secreted modular protein containing several domains typically present in extracellular matrix proteins (EMI, WAP, FNIII, Ca2+ -binding EGF-like, SEA and ZP domains). By in situ hybridization we find that during embryonic development expression of the Umodl1 gene is detectable only in the olfactory epithelium and vomeronasal organ starting at embryonic day 16.5. At this stage, Umodl1 expression within the olfactory epithelium is punctate, and is restricted to only some of the sensory neurons. At birth and postnatally, expression in these organs continues and involves more neurons. Kallmann syndrome is a genetic disease in which olfactory axons fail to connect to target neurons in the bulb. We tested whether olfactorin might be responsible for an autosomal form of this disease and show that this is not the case. However, based on its domain composition and on the expression in olfactory neurons we suggest that olfactorin may play a role in correct olfactory axon navigation to the brain.