Early tests with monkeys are far from perfect. The cells only repaired repaired 16 percent of damaged tissue, and the monkeys still needed drugs to prevent their bodies from rejecting donor cells (in theory, that shouldn't be necessary with this technique). The simians also developed irregular heartbeats, although those might only be temporary. If the team can refine its technique, though, there could be a time when a heart attack doesn't leave you quite so weakened as it does today.