Immunohistochemical analysis (IHC), followed by optical microscopy, was performed to visualize in situ the BCRP-rich areas in the sections of the kidney, liver, and the small and large intestine (Figure 1A) of DBA/2 mice bearing a S91 melanoma tumor at an advanced stage of growth, i.e., of a mass greater than 1 g and volume ≥1000 mm3. The gene discussed is ABCG2; the disease is melanoma.