MDK and central nervous system cancer: GBM cells from all four clusters expressed up to 10 different MDK/PTN receptor genes (Figure 6C), suggesting that these GBM cells can respond to MDK and PTN cytokines produced by the glioma microenvironment [65] and/or produced auto-/ paracrine by GBM cells, as demonstrated for GBM-34, -49, -109, and -228.