A single-cell snapshot of cell-fate decisions.

Critical steps in the specification of embryonic cell lineages occur after implantation, but gaining insight into the molecular details of these cellular processes in vivo has been challenging. Jin and co-workers now report the transcriptomic signatures and molecular heterogeneity of more than 600 single cells from mouse embryos at days 5.5 and 6.5, advancing our understanding of how early embryonic cells make cell-fate decisions into mesoderm and endoderm lineages.