ApoM (apolipoprotein M, also designated G3a or NG20), with 188-amino acid protein(about 21kDa), is an apolipoprotein and member of the lipocalin protein family. The Apo-proteins are involved in the specific binding of cellular receptors, the regulation of lipolytic enzymes, and the process of lipid exchange. The encoded protein is secreted through the plasma membrane but remains membrane-bound, where it is involved in lipid transport. The N-terminal region of Apo-M contains hydrophobic residues that may promote association with the phospholipid layer of lipoprotein particles. In vitro, Apo-M is glycosylated when translated in the presence of microsomes, and remains associated with the microsomes after carbonate treatment. Apo-M is expressed in liver and kidney, and is secreted into the bloodstream in HDLs, and also found in triglyceride-rich lipoproteins and LDLs.