SMPD1 and sphingolipidosis: Types A and B Niemann-Pick disease are autosomal-recessive sphingolipidosis caused by mutations in the sphingomyelin phosphodiesterase 1 gene (SMPD1, MIM# 607608, GenBank# M81780.1) resulting in lysosomal acid sphingomyelinase (ASM, E.C. 3.1.4.12) malfunction.