Subsequently, in several non-P-gp MDR tumour cell lines high levels of other members of the ATP-binding cassette (ABC) transporter superfamily (Higgins, 1992) were identified: the Multidrug Resistance Protein 1 (MRP1; ABCC1; reviewed in Cole and Deeley (1998) and the Breast Cancer Resistance Protein (BCRP; ABCG2) (Doyle et al, 1998). This evidence concerns the gene PGP and neoplasm.