Membrane proteins from MCF-7, MCF-7/AdrVp1000 (A) or MCF-7/Mito (B) cells were photoaffinity labelled with IAARh123 in the absence or presence of a molar excess (300–1000-fold) of mitoxantrone (Mitox), vincristine (VCR), doxorubicin (DOX), cisplatin (Cis-pl), VP-16, taxol or methotrexate (MTX). The relative photolabelling signal of wild-type and mutant ABCG2 with IAARh123 in the presence of these drugs is also shown. The photoaffinity labelling results show a representative experiment that was repeated at least three times. The bar graphs show the relative decrease in the photoaffinity labelling of ABCG2 in the presence of increasing molar concentrations of the drugs averaged between three different independent experiments.