ABCG2 and cancer: The major ABC proteins that are widely accepted to be responsible for the MDR phenotype of cancer cells are P-glycoprotien (P-gp, also called MDR1 or ABCB1), multidrug resistance proteins (MRPs, also called ABCCs) and breast cancer resistance protein (BCRP, also called ABCG2, BCRP, MXR or ABCP) with each having promiscuous and overlapping substrate recognition spectra (9).