The ATP-binding cassette (ABC) transporter superfamily contains membrane-embedded proteins that are involved in the transport of a wide variety of substrates. From all the identified transporters, three are associated with multidrug resistance (MDR): P-glycoprotein (P-gp, ABCB1), the MDR-associated protein-1 (MRP1, ABCC1), and the breast cancer resistance protein (BCRP, ABCG2). ABCG2/BCRP emerged as an important protein involved in MDR and the focus of MDR studies shifted to this protein. ABCG2 (ABC subfamily G member 2) was simultaneously discovered by three different groups in 1998, and is therefore known as ABCP for its identification in human placenta, BCRP for its isolation from the breast cancer cell line MCF-7/AdrVp, and MXR for its identification in the mitoxantrone (MX) resistant colon carcinoma cell line S1-M1-80. The human ABCG2 protein is a half-transporter composed of 665 amino acids with a molecular weight of 72 kDa. 
Unlike other ABC transporters, ABCG2 is a half-transporter with a single nucleotide-binding domain and one membrane-spanning domain containing six transmembrane segments, which is supposed to form homodimers or homomultimers to become functional. Multiple physiological functions have been considered for ABCG2, but in general its tissue localization appears to play a protective role against various xenobiotics limiting their oral absorption. High ABCG2 expression has been found especially in the placenta and intestine, but also in the brain endothelium, prostate, testes, ovaries, liver, adrenal gland, uterus, and central nervous system (CNS). ABCG2 recognizes a broad variety of positively and negatively charged substances and is resistant to most topoisomerase I or II inhibitors such as topotecan or doxorubicin explaining failures of cancer therapy.

References

1.Pe?a-Solórzano D,et al. Med Res Rev. 2017 Sep;37(5):987-1050.