MET and neoplasm: Furthermore, we show that MET signaling, a key mesenchymal and cell motility pathway, is activated exclusively in extreme mesenchymal tumor cells of pericyte-poor tumors, and fueled by glioma-associated macrophages that express the ligand HGF, marking a shift in the generally hardwired OPC-like, PDGFB-driven RCAS/tv-a murine glioma model36 toward the mesenchymal subtype upon pericyte deprivation.