SEQUENCE-SPECIFIC DNA binding activators of gene transcription may be assisted by SWI2(SNF2)^1,2, which contains a DNA-depen-dent ATPase domain³. We have isolated a human complementary DNA encoding a 205K nuclear protein, BRG1, that contains extensive homology to SWI2 and Drosophila brahma^4,5. We report here that a SWI2/BRG1 chimaera with the DNA-dependent ATPase domain replaced by corresponding human sequence restored normal mitotic growth and capacity for transcriptional activation to swi2& minus; yeast cells. Point mutation of the conserved ATP binding site lysine abolished this complementation. This mutation in SW12 exerted a dominant negative effect on transcription in yeast. A lysine to arginine substitution at the corresponding residue of BRG1 also generated a transcriptional dominant negative in human cells. BRG1 is exclusively nuclear and present in a high M_r complex of about 2 & times; 10⁶. These results show that the SWI2 family DNA-dependent ATPase domain has functional con-servation between yeast and humans and suggest that a SWI/SNF protein complex is required for the activation of selective mammalian genes.