Apolipoprotein A1 (Apo A1) is the major protein component of high density lipoprotein (HDL) in plasma. Chylomicrons secreted from the intestinal enterocyte also contain Apo A1, but it is quickly transferred to HDL in the bloodstream. The protein has a specific role in lipid metabolism. It promotes fat efflux, including cholesterol, from tissues to the liver for excretion. It is a cofactor for lecithin cholesterolacyltransferase (LCAT) which is responsible for the formation of most plasma cholesteryl esters. Apo A1 was also isolated as a prostacyclin (PGI2) stabilizing factor, and thus may have an anticlotting effect. Apo A1 is often used as a biomarker for prediction of cardiovascular diseases and the ratio Apo B-100/Apo A1 has been reported as a stronger predictor for the risk of myocardial infarction than any other lipid measurement. Defects in the gene encoding it are associated with HDL deficiencies, including Tangier disease, and with systemic non-neuropathic amyloidosis.