<unk>  has an autosomal recessive pattern of inheritance. Mutations in the SMPD1 gene cause<unk>  types A and B. They produce a deficiency in the activity of the lysosomal enzyme acid sphingomyelinase, that breaks down the lipid sphingomyelin. Mutations in NPC1 or NPC2 cause<unk> , type C (NPC), which affects a protein used to transport lipids. Type D originally was separated from type C to delineate a group of patients with otherwise identical disorders who shared a common Nova Scotian ancestry. Patients in this group are known to share a specific mutation in the NPC1 gene, so NPC is used for both groups. Before the molecular defects were described, the terms<unk><unk><unk>  type I" and<unk><unk><unk>  type II" were proposed to separate the high- and low-sphingomyelin forms of the<unk>  in the early 1980s.<unk>  is inherited in an autosomal recessive pattern, which means both copies, or both alleles of the gene, must be defective to cause the<unk> . "Defective" means they are altered in a way that impairs their function. Most often, the parents of a child with an autosomal recessive disorder are carriers: they have one copy of the altered gene, but are not affected because the other copy produces the enzyme. If both parents are carriers, each pregnancy has a 25% chance of producing an affected child. Genetic counseling and genetic testing are recommended for families who may be carriers of the<unk> .
Niemann–Pick disease
Wilson's disease
Wilson–Mikity syndrome
Haemochromatosis type 3
Mowat–Wilson syndrome
Hemochromatosis type 4
Gaucher's disease
Maroteaux–Lamy syndrome