 ApoD also contains 5 cysteine residues, 4 of which are involved in intra-molecular disulfide bonds.Apolipoprotein D (Apo-D) is a component of high-density lipoprotein that has no marked similarity to other apolipoprotein sequences.It has a high degree of homology to plasma retinol-binding protein and other members of the alpha 2 microglobulin protein superfamily of carrier proteins, also known as lipocalins.