Second, the markedly increased number of lysosomes in ABCG2-overexpressing MDR MCF-7/MR cells and in various other MDR cancer cells relatively to their drug naïve cancer cells as well as normal tissues, renders these MDR cells much more vulnerable to lower light intensities due to a substantial increase in lysosomal sequestration of photosensitizers. Here, ABCG2 is linked to cancer.