NPC1 and lysosomal storage disease: Niemann-Pick type C disease is a recessive lysosomal storage disease that is caused by a mutation of the intracellular cholesterol transporters NPC1 (in 95% of cases) or NPC2 (in 5% of cases), in which un-esterified cholesterol and glycolipids accumulate in lysosomes and late endosomes in the liver, spleen, and brain.