Histone post-translational modifications are critical for establishing chromatin environments that facilitate or inhibit gene expression. Such organization is especially important during developmental processes, as failure to activate or repress lineage specific expression programs leads to embryonic defects and often death (11, 12). Histone acetylation is a key regulator of chromatin accessibility, but we still have much to learn about the roles of specific HATs or HDACS in developmental or disease processes. Our lab has used genetic approaches in mice and in ES cells to define the functions of Gcn5, the first transcription-related HAT to be identified (1). Our previous studies demonstrated that Gcn5 is essential for normal mouse development (5, 6). We also discovered that deletion of Gcn5 leads to loss of a deubiquitylase (DUB) module from the Gcn5- containing SAGA complex and that this module has an unexpected role in telomere maintenance (7). Additional studies in mouse embryonic stem (ES) cells revealed that Gcn5 is an important co-activator for Myc and E2F1 in the regulation of a self-renewal network in ES cells and in reprogramming of somatic cells to a stem cell state (Hirsch et al, submitted). We hypothesize that Gcn5 also regulates critical gene expression programs during development, as reflected by the severity of the phenotypes of Gcn5 mutant embryos. In Aim 1 of this proposal, we propose both genomic and single cell level experiments to define developmental programs that require Gcn5 during ES cell differentiation in vitro. In Aim 2, we propose genome editing approaches to define the functions of a highly conserved bromodomain in Gcn5, which likely serves to promote Gcn5 interactions with acetylated lysines in histones and other proteins important for normal development. Large-scale cancer genomic analyses indicate that Gcn5 is overexpressed in a number of cancers (The Cancer Genome Atlas). Specific SAGA components are also linked to SCA7, a debilitating neurodegenerative disease (9, 10). In the longer term, our definition of the functions of Gcn5 during development will provide new insights into how Gcn5 and SAGA contribute to oncogenesis and neurodegeneration, as well as new therapeutic options for these conditions.