Fig. 7. Multi-step model of EPI/PrE lineage formation. Initial (16- to 32-cell stage) overlapping expression of lineage-specific transcription factors is followed by a progression towards a stabilised salt-and-pepper pattern of PrE (Gata6) and EPI (Nanog) expression (32- to 64-cell stage). Subsequently, stabilised Nanog or Gata6 expression biases cells towards a particular fate (64-cell stage) and cell sorting proceeds, although positional signals (arrows) are still required to complete specification.