ABCG2 and breast cancer: The efflux of PpIX is mediated via another ABC transporter, ABCG2 (also known as the breast cancer resistance protein owing to its ability to confer drug resistance through its membrane transport function).15,22 Not surprisingly, cell studies have shown a negative correlation between the expression of ABCG2 protein and PpIX accumulation.15 In human bladder cancer tissue, reduced levels of ABCG2 expression correlated with increased 5-ALA-induced PpIX fluorescence.14