In the 23 patients with photosensitivity that lacked apparent causes, lower ABCG2 function was significantly correlated with higher erythrocyte protoporphyrin levels (P = 0.012; Fig. 1), suggesting that ABCG2 dysfunction can lead to the accumulation of protoporphyrin in humans. This evidence concerns the gene ABCG2 and phototoxic dermatitis.