For the first time, human stem cells have been converted into insulin-producing pancreatic β cells. Douglas Melton of the Harvard Stem Cell Institute and colleagues report on a complex process that over 35 days produces some 200 million β cells, which would be enough, in theory, to treat a human patient. Transplanted into mice, the β cells work, secreting human insulin and improving hyperglycemia in diabetic mice. In human diabetics, these cells might still be destroyed in the person’s immune system, like the original β cells, but recipients of transplanted cadaveric human tissue transplants have been insulin-free for five years – a procedure that is limited owing to the scarcity of suitable donor tissue. While not yet a cure, this is the first example of making functional, transplantable insulin-producing human cells, and of the importance of continuing stem-cell work.