In eukaryotic cells, genomic DNA is packaged into chromatin that encodes epigenetic information and maintain genome integrity. Chromatin is further organized into distinct functional domains, such as heterochromatin and euchromatin, that contain different post-translational histone modifications (PTM). How different chromatin states are inherited during S phase of the cell cycle is one of the most challenging questions in the chromatin and epigenetic fields. The “first” step in this complex process is the assembly of replicated DNA into nucleosomes using both parental and newly-synthesized histones in a process that is tightly coupled to ongoing DNA synthesis. We have been studying how nucleosomes are formed following DNA replication and have made multiple major contributions to this process. However, how parental histone (H3-H4)2 tetramers, the primary carrier of epigenetic modifications, are transferred to replicating DNA is still poorly understood, which hinders our understanding of the transmission of epigenetic information into daughter cells. The major challenge to understanding parental histone (H3-H4)2 assembly is a lack of methods to track this process. We have developed the eSPAN (enrichment and Sequencing Protein- Associated Nascent DNA) method that can discern whether a protein binds to leading or lagging strands of DNA replication forks in both yeast and mouse embryonic stem (ES) cells. This method makes it possible to identify factors that function in nucleosome assembly of parental histone (H3- H4)2. Moreover, we discovered that cells defective in parental histone transfer compromise the repression of endogenous retrovirus (ERVs), repetitive DNA elements that are normally silenced via a heterochromatin-based mechanism. Others show that ERV reactivation in cancer cells leads to increased response to immunotherapy. In this proposal, we will elucidate the molecular mechanisms whereby parental (H3-H4)2 are reassembled into nucleosomes following DNA replication in yeast and mouse ES cells and determine how deficiencies in this process impact ERV silencing. Together, these studies will address fundamental questions regarding chromatin replication and epigenetic inheritance, while also providing novel insights into a major epigenetic mechanism that boosts the response of cancer cells to immunotherapy.