The μ2 chain of the clathrin-associated adaptor complex AP-2 is a member of the adaptor medium chain family, a group of proteins involved in the sorting of integral membrane proteins in endocytic/exocytic pathways. Here, we report the cloning of the (MMU)CLAPM1 gene encoding the murine μ2 chain, the first member of the family for which this information has become available. The μ2 gene is approximately 8.5kb long and is organized into 12 exons and 11 introns. Two transcripts are generated by alternative splicing of exon 5, a mini-exon of only six nucleotides. Proteins encoded by both transcripts are capable of interacting with tyrosine-based sorting signals, suggesting that they are functionally equivalent. The μ2 gene is localized to the proximal region of mouse chromosome 16, which is syntenic to the proximal region of human chromosome 3. The isolation and characterization of the μ2 gene should be instrumental for future studies of the genetics and physiological role of the adaptor medium chains in mammals.