DES and neoplasm: These results suggest that, when Egfr signaling is interrupted in perivascular precursor cells, as evidenced by the loss of pEgfr staining in the precursor NG2-positive pericytes of Hb-egf mutant tumors (Figs. 7 H and I), either those partially differentiated Desmin-negative pericyte precursors do not mature into fully differentiated Desmin-positive pericytes in the first place or such mature Desmin-positive pericytes are not stably maintained in association with the tumor vasculature.