Cloning of mice to six generations

Mice have been cloned by nuclear transfer into enucleated oocytes, and here we describe the reiterative cloning of mice to four and six generations in two independent lines. Successive generations showed no signs of premature ageing, as judged by gross behavioural parameters, and there was no evidence of shortening of telomeres at the ends of chromosome, normally an indicator of cellular senescence--in fact, these appeared to increase slightly in length. This increase is surprising, given that the number of mitotic divisions greatly exceeds that of sexually produced animals and that any deleterious effects of cloning might be expected to be amplified in sequentially cloned mice. Our results offer a new approach to the study of organismal ageing.