Genetic testing of LDLR, APOB, and PCSK9 in those with extremely high LDL-c (e.g., >250 mg/dl) early onset ASCVD, xanthomas and a family history of severe hypercholesterolemia will reveal a causal mutation >80% of the time, while those with less severe presentations (e.g., absence of xanthomas) may only have a positive genetic test ~50% of the time. Here, PCSK9 is linked to xanthoma.