SMPD1 and Niemann-Pick disease type C: These diseases are currently categorized into two groups: (1) acid sphingomyelinase-deficient Niemann-Pick disease (ASM-deficient NPD), either type A or type B (NP-A or NP-B), resulting from mutations in the SMPD1 gene; and (2) Niemann-Pick disease type C (NP-C), which also includes type D, resulting from mutations in either the NPC1 or the NPC2 gene [1].