Werner syndrome, a rare but highly informative premature ageing syndrome, is caused by mutation of the human WRN gene (Yu et al. 1996) which encodes a large protein (hWRN) possessing both helicase and exonuclease activities (Gray et al. 1997; Huang et al. 1998; Shen et al. 1998). This evidence concerns the gene WRN and Werner syndrome.