that differ between cell types and that have distinct functions23,24 (Fig. 1a). Mice deficient in either of the two ATPases have different phenotypes. Maternal Brg1 is required for zygotic genome activation in a two-cell-stage embryo25, and zygotic Brg1 is essential for both the survival and the proliferation of the cells of the inner cell mass and the trophoblast26 (Table 1 and Fig. 2). By contrast, Brm-knockout mice develop normally, albeit with a slight increase in body mass27. Consistent with the fact that lethality occurs near the time that the embryo is implanted in the uterine wall (at peri-implantation), recent studies have demonstrated that BAF complexes have a crucial role in maintaining the self-renewal and pluripotency of mouse embryonic stem cells (ESCs)28–31. Mouse ESCs produce a complex called esBAF, which is characterized by containing BRG1 but not BRM, and BAF155 but not BAF170 (refs 28, 29); this complex regulates the core pluripotency transcriptional network of mouse ESCs32. So far, complexes containing BAF155 and not BAF 170 have not been found in other cell types. In addition to its role in transcriptional regulation during early embryogenesis, a homologue of BRG1 was also identified in a screen for genes that are essential for
