One of the first lineages to differentiate during mammalian development is the extra-embryonic primitive endoderm (PrE). Its specification was originally thought to occur in response to positional signals, but a recent model suggests that PrE precursors develop randomly within the inner cell mass before segregating from the pluripotent epiblast (EPI). Now, Plusa and colleagues propose a multi-step process for PrE formation in mouse embryos that includes features of both models (see p. 3081). In their study, which is the first to use live imaging to investigate PrE formation,the researchers express a marker gene under the control of regulatory elements of Pdgfra - a novel PrE marker - and analyse lineage-specific markers. Their results suggest that lineage-specific markers are initially expressed in a random, overlapping manner. Then, a gradual progression towards the mutually exclusive expression of PrE and EPI precursors occurs that is partly aided by positional signals, before cell sorting, which involves selective apoptosis and other cell behaviours, completes PrE formation.