PCSK9 and familial hypercholesterolemia: Hypercholesterolemia is caused by disruption of genes which are critical to LDL catabolism, mainly the LDLR gene (MIM# 606945), the APOB gene (MIM# 107730) and the already mentioned gain-of-function mutations in PCSK9 (MIM# 607786), the latter being responsible for a small percentage (<2%) of the cases of FH overall.