EGFR and glioblastoma: This effect was likely to be caused by HO-1-mediated degradation of PpIX, because selective pharmacological inhibition of HO-1 by SnPP restored PpIX fluorescence in GBM cells, independently of their quantitative expression of EGFR, as shown by Flow Cytometry (Fig. 5a1) and immunostaining (Fig. 5a2).