ABCB6 belongs to the B [MDR/TAP] sub-family of ATP-binding cassette transporters. The B subfamily consists of 11 members, with functions ranging from peptide transport (ABCB2-3) to secretion of bile acids (ABCB11). ABCB1-MDR1, the first human ABC transporter cloned and characterized through its ability to confer an MDR phenotype to cancer cells, stands out among ABC transporters by conferring the highest resistance against the greatest variety of compounds. 

Despite considerable advances in drug discovery, resistance to chemotherapy confounds the effective treatment of patients. Cancer cells can become resistant to a single drug or they may acquire broad cross-resistance to mechanistically and structurally unrelated drugs (multidrug resistance (MDR)). ATP-binding cassette (ABC) proteins, present in most living organisms from prokaryotes to mammals, include the best known mediators of MDR. In particular, MDR pumps ABCB1 (MDR1-Pgp), ABCC1-MRP1, and ABCG2-MXR actively extrude many types of drugs from cancer cells, thereby conferring resistance to those agents. Known substrates of Pgp include natural-product antineoplastics such as anthracyclines, vinca alkaloids, taxanes, and epipodophyllotoxins. Pgp expression, frequently detected in human solid and hematological cancers, is a marker of chemoresistance or decreased survival in leukemias, lymphomas, osteosarcomas, small cell lung cancers, and breast cancers, among other malignancies. Thus, innate or acquired expression of Pgp is a major problem in cancer chemotherapy.