Midkine exhibits neurite outgrowth-promoting activity and may play a role in nervous system development and/or maintenance. Its expression is predominant only for a short period from approximately one-half to two-thirds of the way through gestation; before and after that, it is barely detectable. Midkine was first found in differentiating mouse teratocarcinoma cells. It has neurotrophic activities and is mitogenic to certain, but not to all, fibroblast cell lines.