Apolipoproteins are protein components of plasma lipoproteins. Apolipoprotein A-I (ApoA1) promotes cholesterol efflux from tissues to the liver for excretion. ApoA1 is the major protein component of high density lipoprotein (HDL) in the plasma. It can function as a cofactor for lecithin cholesterolacyltransferase, which is responsible for the formation of most plasma cholesteryl esters. AI-BP (Apolipoprotein A-I-binding protein), also known as YjeF N-terminal domain-containing protein 1, is a 288 amino acid secreted protein that binds ApoA1, ApoA2 and HDL. Individuals with impaired renal function show an increased rate of AI-BP excretion, indicating that it is normally reabsorbed within the kidney tubules. AI-BP belongs to the YjeF N-terminal domain protein family, which includes proteins that are frequently involved in oogenesis and spermatogenesis. There are two isoforms of AI-BP that are produced as a result of alternative splicing events.