Apolipoprotein M is an apolipoprotein and member of the lipocalin protein family. It is found associated with high density lipoproteins and to a lesser extent with low density lipoproteins and triglyceride-rich lipoproteins. The encoded protein is secreted through the plasma membrane but remains membrane-bound, where it is involved in lipid transport. Two transcript variants encoding two different isoforms have been found for this gene, but only one of them has been fully characterized.The deduced 188-amino acid protein has a calculated molecular mass of about 21 kD. It contains a potential N-glycosylation site, 2 possible disulfide bridges, and a hydrophobic alpha-helical signal peptide that is retained in the mature protein. Sequence analysis revealed 79% identity between the human and mouse proteins.