Efflux of exogenous PPIX by ABCG2. K562 cells or K562 cells engineered to overexpress ABCG2 (K562/ABCG2) are incubated with Hoechst 33342 (A, B) or PPIX (C, D). Cells also were coincubated with the ABCG2 inhibitor Ko143 (B, D). Shaded area indicates K562 cells; solid line, K562/ABCG2 cells.