In this study, we observed noticeable reductions in the mRNA and protein expression of ABCG2 in cells following the addition of the combined ALA and hemin, indicating that hemin might play a key role in repressing the expression of ABCG2 to prevent the efflux of intracellular PpIX, thereby participating to effectively increase the PpIX-mediated cellular toxicity in A549 lung cancer cells [33]. The gene discussed is ABCG2; the disease is lung cancer.