With -1000 genes, the odorant receptor (OR) gene repertoire is the largest gene family in the mouse genome. Here we have established a 129/Sv BAC contig for mouse OR gene cluster 7 (Olfr7) on Chromosome (Chr) 9. The assembled approximately 2-Mb contig consists of 75 BACs and may contain as many as 100 OR genes, or approximately 10% of the mouse repertoire. Facilitated by the lack of introns in the coding region, we have determined the nucleotide sequence of 37 full-length, 2 partial, and 3 pseudo coding regions. These 42 OR genes and 3 additional OR genes previously mapped to the mouse Olfr7 cluster can be organized into 13 classes based on OR probe cross-hybridizations with 129/Sv mouse genomic DNA. OR genes belonging to the same class tend to be located next to each other within the cluster. Comparison of published full-length mouse and rat OR coding sequences with those identified here shows that the Olfr7 OR genes are highly related to each other, clustering on two major branches of an unrooted phylogenetic tree. Eight ORs contain an unusual NXC sequon at the amino-terminal extracellular domain that may represent a novel N-linked glycosylation site. The BAC contig presented here provides the substrate for sequencing of the cluster.