ABCB1 and neoplasm: Identification of this (these) reactive metabolite(s) as well as investigating its (their) rate of cellular accumulation, level of nuclear retention and rate of active export by MDR exporting pumps (P-glycoprotein, MRP1) (Tarasiuk et al, 1989, 2004; Tkaczyk-Gobis et al, 2001) is needed in order to clarify the role of reductive activation of DOX by cellular oxidoreductases in the mode of action of this drug in regard to resistant tumour cells.