Pancreatic islets contain several different types of hormone-producing(endocrine) cells, damage to which can lead to disease, such as diabetes. During embryonic development, all pancreatic endocrine progenitor cells express the transcription factor Ngn3, but do individual Ngn3+progenitors give rise to multiple endocrine cell types or only to one? The answer, claim Desgraz and Herrera, is the latter (see p. 3567). Using a genetic system called MADM, the researchers generated transgenic mosaic mice in which very few Ngn3+ cells are fluorescently labelled, and then traced the fate of individual Ngn3+ cells. They found that at birth, each Ngn3+ cell had turned into a single endocrine cell. In adult mice, small homogeneous clusters of Ngn3+-derived labelled cells exist, which indicates low cell proliferation. These findings suggest that the Ngn3+ progenitors are heterogeneous, as they are unipotent but give rise to multiple cell types, and showcase the potential of MADM for investigating cell fate specification events in vivo at the single-cell level.