SMPD1 and nasopharyngeal carcinoma: The first is due to complete or partial deficiency of acid sphingomyelinase, resulting from mutations in the SMPD1 gene with the clinical phenotypes of NP type A and B. NPC is characterized by unique abnormalities of intracellular transport of endocytosed cholesterol with sequestration of unesterified cholesterol in lysosomes and late endosomes, but the function of the affected proteins (NPC1 and NPC2, encoded by NPC1 and NPC2 genes, respectively) are not precisely elucidated (Ory 2004).