PROJECT SUMMARY / ABSTRACT DNA encodes all of the information required to generate and maintain each cell within the body. However, DNA alone is not sufficient and requires the genome to be labeled as active or inactive in a process termed epigenetics by modifications to DNA itself or to histones around which all DNA is wrapped. These epigenetic modifications are well established regulators of cellular development and differentiation, and their function is essential for proper retinal development. While modifications to histone tails such as acetylation and methylation have been extensively studied, we have not yet explored the roles of histone variants in the retina. In fact, histone H3 has two isoforms H3.1/2 and H3.3, which are incorporated in a cellular replication dependent and independent manner respectively. Recent evidence has emerged that mutations within H3.3 cause syndromic disorders, which include severe neurodevelopmental issues, including in some cases retina degeneration and vision loss. In this proposal, we aim to investigate the role of H3.3 in retinal cell fate specification and differentiation. We have preliminary evidence that a mouse model lacking H3.3 in the developing eye causes loss of a subset of retinal cell types. Additional data suggests H3.3 deposition is tightly regulated early in development to control expression of an essential subset of genes. Therefore, in Aim 1 we will complete a full phenotypic characterization of retinal development upon loss of H3.3 from retinal progenitors and post-mitotic neurons. In Aim 2, we will investigate the molecular role of H3.3 in the establishment of the mature retinal epigenome. The results of this study will deliver the first in depth investigation of retinal phenotype after depletion of H3.3 and define the role of H3.3 in retinal epigenetic remodeling and homeostasis. A better understanding of the role of H3.3 in the retina is essential to better understand retinal development and for the design of treatment paradigms for patients with H3.3 mutations.