Pleiotrophin and Midkine are structurally related heparin-binding neurotrophic factors, whose expression is developmentally regulated. The expression pattern of these neurotrophic factors suggests function in neurogenesis, cell migration, secondary organogenetic induction, and mesoderm epithelial interaction. The expression of PTN increases during the process of brain embryogenesis, and reaches maximum levels at time of birth. The physiological roles of PTN and Midkine are largely unknown, but these neurotrophins have been implicated in the pathogenesis of neuroblastomas.