Apoliprotein M (APOM) is a plasma protein usually associated with HDLs and to a lesser extent, with LDLs. APOM could be classed as a lipocalin (LCN) because it shares the structurally conserved beta-barrel, which in many LCNs, binds hydrophobic ligands. Mature APOM retains its signal peptide, which serves as a lipid anchor to attach it to lipoproteins, thereby keeping it in the circulation (Christoffersen et al. 2008). APOM is able to bind retinoids such as retinol, all-trans-retinoate and 9-cis-retinoate with low affinity although they may not be the natural ligands (Ahnstrom et al. 2007, Dahlback & Nielsen 2009). APOM does not bind cholesterol, vitamin K or arachidonate (Ahnstrom et al. 2007).