In summary, FH is typically caused by mutations in the either the LDL receptors, in ApoB, PCSK9, LDL RAP1, or in other as-yet unidentified genes.  Persons with severe FH have an LDL >300 mg/dL without symptomatic CHD or an LDL >200 mg/dL with symptomatic CHD.  Severe FH occurs in between 1 in 10,000 and 1 in 50,000 people in the United States, with the pathology resulting in lifelong elevated LDL and CHD occurring very prematurely.  The treatment of severe FH involves aggressive LDL reduction and in some cases may require heroic measures like LDL apheresis.