Ieda and colleagues1 recently showed that the expression of specific transcription factors in the mouse fibroblast can instigate the cell's transdifferentiation into a cardiomyocyte, as evidenced by the expression of green fluorescent protein (GFP), which has been engineered in such a way that it is turned on only in cardiomyocytes. Reprogrammed cells attained different tiers of cardiomyocyte identity — for example, 20% had cardiac reporter expression through a GFP transgene, 4% had cardiac expression of sarcomere genes (e.g., troponin T), and 1% showed spontaneous contractions and other functional properties of cardiomyocytes.