Developmental Biology Pluripotent embryonic stem cells (ESCs) can differentiate into any adult cell type; however, aggregates of these cells do not mimic embryonic architecture when grown in culture. To see whether mouse ESCs and their extraembryonic counterparts, trophoblast stem cells (TSCs), can recapitulate normal development, Harrison et al. combined ESCs and TSCs in an extracellular matrix culture (see the Perspective by Pera). The resultant “ETS-embryos” displayed considerable resemblance to normal embryos, even specifying mesoderm and primordial germ cells at the boundary between embryonic and extraembryonic compartments. These ETS-embryos are a genetically tractable tool for studying mammalian embryogenesis. Science , this issue p. [eaal1810][1]; see also p. [137][2] [1]: /lookup/doi/10.1126/science.aal1810 [2]: /lookup/doi/10.1126/science.aan1495