ABCG2 and neoplasm: Despite the BCRP-expressing cells having a similar level of resistance to STX140 and MXR in vitro, STX140 was highly active in vivo in both MCF-7wt and MCF-7.MR tumours, significantly inhibiting their growth, whereas MXR, a chemotherapeutic agent in clinical use, inhibited the growth of the wild-type xenografts, but was inactive in those which expressed BCRP.