ABCG2 and breast cancer: Some membrane transport proteins are known to be involved in drug resistance by altering drug transport and pumping drugs out of tumor cells [61,64,65], such as ABCB1 (ATP-Binding Cassette Subfamily B Member 1), which expression determines a tumor phenotype resistant to several drugs, such as vinca alkaloids, epipodophyllotoxins, taxanes and anthracyclines, and ABCG2 (ATP-Binding Cassette Subfamily G Member 2 (JR Blood Group)), a protein associated with breast cancer resistance [66].