There had been a few known ligands for PTPRZ1 including pleiotrophin (PTN), midkine (MDK), neuronal cell adhesion molecule (NRCAM), interleukin 34 (IL34), Contactin 1 (CNTN1), NCAN, PGCB.[45, 46] For example, in glioblastoma multiforme (GBM), tumor‐associated macrophage (TAM) secreting PTN binds to PTPRZ1 and thus maintains sustained tyrosine phosphorylation of downstream substrates, including β‐catenin and receptor tyrosine kinases (RTKs), thereby activating oncogenic pathways.[47] By disrupting the interaction between PTPRZ1 and its inhibitory ligand would prevent cancer cell proliferation. Here, MDK is linked to cancer.