According to previous reports, ABCG2 plays an important role in the accumulation of 5‐ALA‐PpIX in cancer cells.28, 29, 30 In the present study, we have clearly demonstrated that combined exposure to 5‐ALA and Ko143 significantly augmented overall PpIX‐FI and reduced cell‐to‐cell variability in breast cancer cell lines. This evidence concerns the gene ABCG2 and breast cancer.