Heart-muscle cells (in close-up, artificially coloured) do not normally proliferate in adult mammals, but scientists are trying to coax them into doing so. Credit: Medimage/SPL Scientists are developing a new wave of gene therapies to regenerate the heart — offering hope for treating heart failure , a debilitating and common condition. The first clinical trial aimed at growing new heart-muscle cells is now under way, and companies are developing at least four other regenerative gene therapies for heart conditions. “These are the first-in-human studies to take regeneration into the clinic,” says Andrew Baker, a gene-therapy researcher at the University of Edinburgh, UK, who is unaffiliated with the efforts. “It’s a very exciting time.” Ranking the risk of heart disease But the excitement is tempered with caution.…