During preimplantation mouse development, the inner cell mass (ICM) 
differentiates into two cell lineages--the epiblast and the primitive endoderm 
(PrE)--whose precursors are identifiable by reciprocal expression of Nanog and 
Gata6, respectively. PrE formation depends on Nanog by a non-cell-autonomous 
mechanism. To decipher early cell- and non-cell-autonomous effects, we performed 
a mosaic knockdown of Nanog and found that this is sufficient to induce a PrE 
fate cell autonomously. Strikingly, in Nanog null embryos, Gata6 expression is 
maintained, showing that initiation of the PrE program is Nanog independent. 
Treatment of Nanog null embryos with pharmacological inhibitors revealed that 
RTK dependency of Gata6 expression is initially direct but later indirect via 
Nanog repression. Moreover, we found that subsequent expression of Sox17 and 
Gata4--later markers of the PrE--depends on the presence of Fgf4 produced by 
Nanog-expressing cells. Thus, our results reveal three distinct phases in the 
PrE differentiation program.