Preimplantation development is crucial for successful implantation and pregnancy in mammals. Compaction, an essential morphological change that occurs in eight-cell-stage embryos, has been extensively studied, but what regulates the preceding cell division stages? On p. 2751, Maekawa and colleagues report that extracellular-signal-regulated kinase (ERK)mitogen-activated protein (MAP) kinase function is needed for these divisions in mouse embryos. They show that inhibition of ERK activation in late two-cell-stage embryos causes reversible arrest in G2 phase at the four-cell stage and that cell-cell adhesion is weaker in these embryos than in control embryos. Their microarray analysis shows that, although most of the changes in gene expression that occur during the four- to eight-cell stages of development occur in the four-cell-stage-arrested embryos, the expression of a subset of genes, including those encoding intercellular adhesion molecules and a set of cell cycle-related genes, is altered. Thus, the researchers conclude,ERK MAP kinase function is essential during the earliest stages of preimplantation development.