An autosomal recessive lipid storage disorder that is characterized by accumulation of cholesterol and sphingomyelins in cells of the viscera and the central nervous system. type c (or c1) and type d are allelic disorders caused by mutation of the npc1 gene, which encodes a protein that mediates intracellular cholesterol transport from lysosomes. clinical signs include hepatosplenomegaly and chronic neurological symptoms. type d is a variant in people with a nova scotia ancestry.