Apolipoprotein A-I: APOA-I. The major protein component of HDL (high density lipoprotein) and a relatively abundant plasma protein. APOA-I is instrumental in promoting the transfer of cholesterol into the liver where it is metabolized and then excreted via the intestine from the body. The gene locus for APOA-I is on chromosome 11q23. A number of genetic variants of APOA1 are known, including ApoA-I Milano.