APOA-I. The main necessary protein part of HDL (high density lipoprotein) and a comparatively abundant plasma necessary protein. APOA-I is instrumental in promoting the transfer of cholesterol levels in to the liver where its metabolized and excreted through the intestine from body. The gene locus for APOA-I is on chromosome 11q23. A number of hereditary variations of APOA1 tend to be understood, including ApoA-I Milano.