The imprinted mouse insulin-like growth factor type 2 receptor (Igf2r) maps to the middle of a gene-rich region in band A2 of mouse chromosome 17. The t(Lub2) chromosome 17 variant contains a small deletion that removes at least seven genes including Igf2r. We have constructed a YAC contig spanning the entire t(Lub2) deletion and created a restriction map that covers 700 kb. The position, transcription orientation, and imprinted status of the genes immediately flanking Igf2r have been assessed. We show here that the Mas gene, which lies 65 kb upstream to Igf2r, contains a novel 5' exon and is not imprinted in adult tissues. We further show that the recently identified Lx1 gene lies immediately downstream and is also expressed from both parental alleles in adult tissues. The remaining genes in this region have previously been shown to be biallelically expressed.