to incoming signaling pathways is brought about through the actions of lineage-specifying transcription factors and specific cell membrane receptors, but other components must be present that are capable of reversing long-standing patterns of epigenetic repression. A biochemical search for proteins binding to accessible NFAT (nuclear factor of activated T cells) sites, which is the terminus of the ubiquitous Ca2+/calcineurin/NFAT signaling pathway, revealed that they bound a complex resembling yeast SWI/SNF, which had been discovered earlier in yeast. However, as the subunits were purified and cloned, it became clear that the mammalian complex (which we initially called the mSWI/SNF complex) was not as close to the yeast complex as we initially thought (4, 14–17, 19). The complexes were assembled combinatorially from the products of at least 29 genes encoded by 15 gene families (36, 48), again highlighting the immense combinatorial diversity and possibly functional diversity encoded within these complexes in much the same way that letters are assembled into words (48). In addition, some of the subunits were found to be tissue-specific, such as those found only in BAF complexes of mature neurons (Fig. 2). For example, BAF53b (an actin-like molecule), BAF45b [a double plant homeodomain (PHD) finger protein], and CREST
