Mutations of the TOP1 gene have been observed in tumour cell lines selected for resistance to camptothecin (Takatani et al, 1997; Pommier et al, 1999; Tsurutani et al, 2002; Woo et al, 2002) or sometimes in clinical specimens (Ohashi et al, 1996), indicating that amino-acid changes could have consequences on cell response to this drug. This evidence concerns the gene TOP1 and neoplasm.