In D. melanogaster, correct body segmental identity is determined by the proper expression patterns of homeotic genes of the Antennapedia complex and the Bithorax complex. Misexpression of homeotic genes leads to segmental transformations and other patterning defects. The expression patterns of genes in the Antennapedia and Bithorax complexes are first established by the actions of the gap and pair-rule groups of genes and are then maintained by the opposing actions of PcG proteins (which are repressive) and TrxG proteins (which are activating). The genes encoding the core homologues of yeast (Saccharomyces cerevisiae) Swi/Snf in D. melanogaster — brahma (brm), osa and moira (mor) — were first identified in screens for suppressors of homeotic transformations caused by mutations in the Polycomb gene17 and were hence classified as TrxG genes. D. melanogaster SWI/SNF proteins are present and function in a multisubunit complex known as Brahma-associated proteins (BAP). Subsequently, analysis of several other subunits of the BAP complex — SNR1 (Snf5-related protein 1; a homologue of mammalian BAF47 and yeast Snf5) and SAYP (supporter of activation of yellow protein; also known as E(Y)3;
