The vomeronasal system mediates pheromonal effects in mammals. We have employed gene targeting technology to introduce mutations in a putative pheromone receptor gene, VR_i2, in the germline of mice. By generating alleles differentially tagged with the histological markers taulacZ and tauGFP, we show that VR_i2 is monoallelically expressed in a given neuron. Axons of VR_i2-expressing neurons converge onto numerous glomeruli in the accessory olfactory bulb. The pattern of axonal projections is complex and variable. This wiring diagram is substantially different from that of the main olfactory system. The projection pattern is disrupted by deleting the coding region of VR_i2, but an unrelated seven-transmembrane protein, the odorant receptor M71, can partially substitute for VR_i2.