Apolipoprotein A-I is the major protein component of high density lipoprotein (HDL) in plasma encoded by the APOA1 gene. It is also found in chylomicrons.and synthesized in the liver and small intestine. It participates in the reverse transport of cholesterol from tissues to the liver for excretion by promoting cholesterol efflux from tissues and by acting as a cofactor for the lecithin cholesterol acyltransferase (LCAT). Defects in APOA1 are a cause of high density lipoprotein deficiency type 2 (HDLD2) also known as familial hypoalphalipoproteinemia (FHA). Defects in APOA1 are also a cause of the low HDL levels observed in high density lipoprotein deficiency type 1 (HDLD1).