PCSK9 and Hypocholesterolemia: Homozygous or compound heterozygous loss-of-function ANGPTL3 mutations cause familial combined hypolipidemia [13]; SAR1B mutations lead to the autosomal recessive chylomicron retention disease (OMIM #246700) with severe hypocholesterolemia and fat malabsorption, but normal TG; and loss-of-function mutations in PCSK9 have been famously associated with depressed serum LDL-C, but not to the degree observed here.