SMPD1 and acid sphingomyelinase deficiency: Acid sphingomyelinase deficiency (ASMD; also known as Niemann–Pick Disease Type A, Type B or Type A/B; OMIM #257200, #607616) is a rare lysosomal storage disorder (LSD) caused by reduced activity of the acid sphingomyelinase enzyme (ASM, EC 3.1.4.12).