Breast cancer resistance protein (BCRP) is a half-molecule ATP-binding cassette transporter that pumps out various anticancer agents such as 7-ethyl-10-hydroxycamptothecin, topotecan and mitoxantrone. We have previously identified three polymorphisms within the BCRP gene, G34A (substituting Met for Val-12), C376T (substituting a stop codon for Gln-126) and C421A (substituting Lys for Gln-141). C421A BCRP-transfected murine fibroblast PA317 cells showed markedly decreased protein expression and low-level drug resistance when compared with wild-type BCRP-transfected cells. In contrast, G34A BCRP-transfected PA317 cells showed a similar protein expression and drug resistance profile to wild-type. The C376T polymorphism would be expected to have a considerable impact as active BCRP protein will not be expressed from a T376 allele. Hence, people with C376T and/or C421A polymorphisms may express low levels of BCRP, resulting in hypersensitivity of normal cells to BCRP-substrate anticancer agents. Estrogens, estrone and 17beta-estradiol, were previously found to restore drug sensitivity levels in BCRP-transduced cells by increasing the cellular accumulation of anticancer agents. BCRP transports sulfated estrogens but not free estrogens and in a series of screening experiments for synthesized and natural estrogenic compounds, several tamoxifen derivatives and phytoestrogens/flavonoids were identified that effectively circumvent BCRP-mediated drug resistance. The kinase inhibitors gefitinib and imatinib mesylate also interact with BCRP. Gefitinib, an inhibitor of epidermal growth factor receptor-tyrosine kinase, inhibits its transporter function and reverses BCRP-mediated drug resistance both in vitro and in vivo. BCRP-transfected human epidermoid carcinoma A431 cells and BCRP-transfected human non-small cell lung cancer PC-9 cells show gefitinib resistance. Imatinib, an inhibitor of BCR-ABL tyrosine kinase, also inhibits BCRP-mediated drug transport. Hence, both functional SNPs and inhibitors of BCRP reduce its transporter function and thus modulate substrate pharmacokinetics and pharmacodynamics.