The cDNA and complete gene encoding mouse acid sphingomyelinase (ASM) has been isolated using a homology screening approach. Comparison with the human sequence shows 81% sequence identity at cDNA level and 82% at the protein level. Homology is markedly reduced in the N-terminal region, especially in the presumed signal peptide. The six-exon gene structure is similar to the human one, except for the position of Alu1 elements. The alternatively used splice site 40 bp downstream of the human exon 2 is not conserved in the mouse gene and accordingly no such alternative splicing was found in mouse.