MK was induced in human neurofibromas, schwannomas, and various nervous system tumors associated with NF1 or NF2; midkine showed an expression pattern overlapping but distinct from its homolog pleiotrophin (PTN).
Pleiotrophin (PTN) is a heparin-binding growth factor with diverse biological activities, the most studied of these being those related to the nervous system, tumor growth and angiogenesis.