Midkine was originally identified as a retinoic acid inducible gene in embryonic carcinoma cells. It is a heparin-binding neurotrophic factor expressed in various cell types during embryogenesis. Midkine promotes cell proliferation, differentiation, survival, and migration. It is involved in a variety of biological processes including neuronal development and angiogenesis. Midkine is also expressed in several carcinomas, suggesting a possible role in tumorigenesis. In addition, Midkine may play a key role in inflammation via the promotion of neutrophil and macrophage chemotaxis and suppression of regulatory T cell expansion. Midkine is a highly basic non-glycosylated protein with 121 amino acid residues that includes ten conserved cysteine residues, which form five intra-chain disulfide bonds. There are several candidate receptors for Midkine including anaplastic lymphoma kinase (ALK), protein-tyrosine phosphatase zeta (PTPzeta), integrins, Notch2, and LRP1. Midkine is also known as neurite growth promoting factor 2 and has been shown to induce neuronal neurite outgrowth in vitro.