Background:  Extracellular signal‐regulated kinase 2 (ERK2) has been implicated in cell proliferation, differentiation, and survival. However, its role in vivo remains to be determined.

Results:  Here we show that the targeted disruption of the mouse ERK2 gene results in embryonic lethality by E11.5 and severe abnormality of the placenta. In these animals, the labyrinthine layer of the placenta is very thin and few foetal blood vessels are observed. ERK2 mutants can be rescued by the transgenic expression of ERK2, demonstrating that these abnormalities are caused by ERK2‐deficiency. Although ERK2‐deficient fetuses are much smaller than wild‐type littermates, this seems to be secondary to malfunction of the placenta. When the placental defect is rescued by tetraploid‐aggregation, ERK2‐deficient foetuses grow as well as littermate controls.

Conclusion:  These observations indicate that ERK2 is essential for placental development and suggest that ERK2 in the trophoblast compartment may be indispensable for the vascularization of the labyrinth.