PCSK9 and Hypercholesterolemia: Familial defects in apolipoprotein B-100 are caused by variants in APOB, familial hypercholesterolemia is caused by variants in the LDL receptor (LDLR), and non-FH/non-FDB hypercholesterolemia is caused by variants in PCSK9. Furthermore, lipid storage disorders other than cerebrotendinous xanthomatosis (CTX) rarely result in neurological damage.