LONDON – New guidelines for stem cell research open the door to extending the legal limit on human embryo research beyond the current 14-day maximum set down 40 years ago. In revised guidelines, the International Society for Stem Cell Research (ISSCR) has moved research on human embryos from category 3, which explicitly bans their study in culture post 14 days in any circumstances, to category 2B, in which research post 14 days would be permissible if there is a clear scientific rationale – and after a thorough specialized review.