In this report, we describe the isolation and analysis of over 1,200 cDNAs representing 419 odorant receptor genes. We screened a mouse olfactory epithelium library with degenerate olfactory receptor probes and obtained 5' end sequences (ESTs) from purified cDNAs. These clones confirm olfactory expression of over 400 olfactory receptors, provide their gene structure, demonstrate that not all olfactory receptors are expressed at the same level and show that most olfactory receptor genes have multiple transcriptional isoforms.