SMPD1 and Niemann-Pick disease type C: The NPD group is now divided into two distinct entities: acid sphingomyelinase-deficient Niemann-Pick disease (ASM-deficient NPD) resulting from mutations in the SMPD1 gene and encompassing type A (OMIM 257200) and type B (OMIM 607616), as well as intermediate forms [67]; and Niemann-Pick disease type C (NPC) and type D, resulting from mutations in either the NPC1 or the NPC2 gene, respectively [68].