The derivation of the primitive endoderm layer from the pluripotent cells of the 
inner cell mass is one of the earliest differentiation and morphogenic events in 
embryonic development. GATA4 and GATA6 are the key transcription factors in the 
formation of extraembryonic endoderms, but their specific contribution to the 
derivation of each endoderm lineage needs clarification. We further analyzed the 
dynamic expression and mutant phenotypes of GATA6 in early mouse embryos. GATA6 
and GATA4 are both expressed in primitive endoderm cells initially. At embryonic 
day (E) 5.0, parietal endoderm cells continue to express both GATA4 and GATA6; 
however, visceral endoderm cells express GATA4 but exhibit a reduced expression 
of GATA6. By and after E5.5, visceral endoderm cells no longer express GATA6. We 
also found that GATA6 null embryos did not form a morphologically recognizable 
primitive endoderm layer, and subsequently failed to form visceral and parietal 
endoderms. Thus, the current study establishes that GATA6 is essential for the 
formation of primitive endoderm, at a much earlier stage then previously 
recognized, and expression of GATA6 discriminates parietal endoderm from 
visceral endoderm lineages.