The mammalian homologues of the BAP complex have similarly widespread roles in development, although at present there is no evidence that mammalian complexes have TrxG-protein-like functions during the specification of segment identity in vertebrates. Studies of BAF complexes in mammals indicate that these complexes undergo progressive changes in subunit composition during the transition from a pluripotent stem cell to a multipotent neuronal progenitor cell to a committed neuron (Fig. 1a). In mammals, the ATPase subunit of the SWI/SNF complex is encoded by two homologues, Brm and Brg1 (brahma-related gene 1). The ATPase is 1 of 12 subunits that seem to be non-exchangeable in vitro. Several subunits are encoded by gene families (for example, BRG1 and BRM are encoded by one gene family, and only one of these is present in each complex), giving rise to a diversity of stable assemblies 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
