Apolipoprotein M (Apo M) is encoded by the APOM gene in humans. The protein encoded by this gene is an apolipoprotein and is a member of the Apo family of proteins. Apo M has been found to be associated with high-density lipoproteins and, to a lesser extent, with low-density lipoproteins and triglyceride-rich lipoproteins.Apo M is secreted through the plasma membrane but remains bound to the membrane and is involved in lipid transport. The APOM gene was found to have 2 transcript variants encoding 2 different isoforms, but only one of them was fully characterized.The average molecular weight of Apo M is 21,253 Da, and the molecular weight of the single isoform is 21,239 Da.