Midkine and its functionally-related protein Pleiotrophin (PTN), are heparin-binding neurotrophic factors that signal through the same receptor. Midkine is involved in a number of biological processes including the migration of neutrophils, inflammatory leukocytes, macrophages, neuronal cells, and osteoclasts. Midkine is a basic, non-glycosylated polypeptide that contains five intra-chain disulfide bonds. There is 87% identity between the human and murine Midkine and approximately 50% identity between human Midkine and human PTN with the conservation of all 10 cysteines.