In addition to the genomic regulation of ABCG2,Imai et al. (2005) also discovered that 17β-estradiol activation of ERα increases topotecan cytotoxicity via a nongenomic pathway through posttranscriptional regulation of ABCG2 in human breast cancer cells (Imai et al., 2005). The gene discussed is ABCG2; the disease is breast carcinoma.