ABCG2 and neoplasm: Members of the ATP-binding cassette superfamily of proteins (ABCB1 (Patel and Mitra, 2001; Gottesman et al, 2002) and ABCG2 (Wierdl et al, 2003)) function to export irinotecan from tumour cells, while drug metabolism via the cytochrome P450 system (CYP3A4 (Kehrer et al, 2002; Xu and Villalona-Calero, 2002)) or glucuronidation (UGT1A1 (Iyer et al, 1999, 2002)) can also occur.