"The difference in cloning rates of male and female stem cells seems likely to involve epigenetics," said Prof Fuchs. Epigenetic modifications are those that affect a gene's function without altering its DNA sequence.

Because one of a female's two X chromosomes (men only have one) is turned off through epigenetic modifications, female cells have to undergo a more complicated epigenetic reprogramming than male ones and this may account for why males are easier to clone.