Apolipoprotein A1 (APOA1) is the main protein component of high density lipoprotein (HDL) in plasma. This protein promotes cholesterol efflux from tissues to the liver for excretion, and it is a cofactor for lecithin cholesterolacyltransferase (LCAT) which is responsible for the formation of most plasma cholesteryl esters. Defects in APOA1 are associated with HDL deficiencies, including Tangier disease, and with systemic nonneuropathic amyloidosis. High level of APOA1 is regarded as protective against atherosclerosis and cardiovascular disease.