The mu 2 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 mu 2 chain, the first member of the family for which this information has become available. The mu 2 gene is approximately 8.5 kb 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 mu 2 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 mu 2 gene should be instrumental for future studies of the genetics and physiological role of the adaptor medium chains in mammals.