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Summary: SAGA (Spt-Ada-Gcn5 acetyltransferase) and ATAC (Ada-two-A-containing) are two related coactivator complexes, sharing the same histone acetyltransferase (HAT) subunit. The HAT activities of SAGA and ATAC are required for metazoan development, but the role of these complexes in RNA polymerase II transcription is less understood. To determine whether SAGA and ATAC have redundant or specific functions, we compare the effects of HAT inactivation in each complex with that of inactivation of either SAGA or ATAC core subunits in mouse embryonic stem cells (ESCs). We show that core subunits of SAGA or ATAC are required for complex assembly and mouse ESC growth and self-renewal. Surprisingly, depletion of HAT module subunits causes a global decrease in histone H3K9 acetylation, but does not result in significant phenotypic or transcriptional defects. Thus, our results indicate that SAGA and ATAC are differentially required for self-renewal of mouse ESCs by regulating transcription through different pathways in a HAT-independent manner.