ABCG2 effluxes endogenous PPIX into medium. K562 or K562/ABCG2 cells were incubated with 1 mM ALA for 7 hours, and the PPIX levels in both medium and within cells were measured by HPLC. The peak of PPIX is indicated in each panel. The peak on the left represents a component in the culture medium (not produced by cells) and serves as an internal control.