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). This evidence concerns the gene HMOX1 and glioblastoma.