ABCG2 and breast carcinoma: The strong time dependence of the activity of ester 2 in the MCF-7+Topo breast carcinoma cells can be a hint at a metabolism of 2 in these cells leading to active illudin M. Similarly, a strong time dependence of compound 2 activity was observed in HT-29 colorectal cancer cells, which are also ABC-transporter expressing multidrug-resistant cells [30].