Apolipoprotein A1 (Apo A1) is the major protein component of high-density lipoprotein (HDL) in plasma. It is also present in chylomicrons secreted by intestinal epithelial cells, but quickly transfers to HDL in the bloodstream. This protein plays specific roles in lipid metabolism, promoting the efflux of fats, including the transport of cholesterol from tissues to the liver for excretion. It acts as a cofactor for lecithin-cholesterol acyltransferase, responsible for the formation of most plasma cholesterol esters. Apo A1 is also a stabilizing factor for prostacyclin, suggesting a potential anticoagulant function. It is commonly used as a biomarker for predicting cardiovascular diseases. The Apo B-100/Apo A1 ratio has been reported as an effective predictor of myocardial infarction risk compared to other lipid measurements. Deletions in the gene encoding Apo A1 are associated with HDL deficiencies, including Tangier disease, and systemic non-neuropathic amyloidosis.