Niemann-Pick disease type A, the most common form, is caused by mutations in the acid sphingomyelinase (ASM) gene SMPD1. Deficiencies of ASM lead to destruction of nerve cells. An infant born with two copies of the same mutation in the SMPD1 gene is normal at birth but becomes ill within a few months; type A disease is severe, and death usually occurs by age 3. Carriers of one type A risk allele are generally considered normal, but preliminary research suggests that a single type A allele, if inherited from the mother, may cause mild symptoms.
Other types of Niemann-Pick disease are less severe than type A; see the National Niemann-Pick Disease Foundation.
More than 100 different mutations are known to occur in the SMPD1 gene. 23andMe tests for the following SNPs that cause Niemann-Pick disease type A: