Further analysis of T1 and T3 tissues by Masson's trichrome and CK18 staining revealed that CK18-positive ccRCC tumor cells were replaced by a collagen-rich tissue (blue staining) in T3 xenografts that ceased responding to sunitinib (T3 non-responder), while collagen levels remained similar between the original (T0) and T1 tissues (Figures 4D–I). This evidence concerns the gene KRT18 and nonpapillary renal cell carcinoma.