After fertilization, a single zygote proceeds through a series of cleavage steps to develop into a multicellular embryo, called a blastocyst. The cells of the blastocyst are capable of generating all adult cell types, a phenomenon known as pluripotency. The inner cell mass (ICM) of the blastocyst can moreover be cultured in a dish as pluripotent embryonic stem cells (ESCs). ESCs have become invaluable tools in regenerative medicine and to study development itself. With 1 in 8 couples experiencing infertility in the UK, it is ever more important to understand the factors contributing to healthy embryo development. Transposable elements (TEs) are parts of our DNA that are currently or historically mobile, -i.e. having the capacity to 'paste' themselves into new places in the genome. Many TE sequences used to be thought of as simply 'junk DNA'; however, we are beginning to understand that TEs have evolved to play new and unexpected roles in development and disease. For example, uncontrolled TE activity has been implicated in neurodegeneration and cancer. However, the expression of many TEs is also high in normal development, suggesting that they may also have beneficial roles in cells. This proposal focuses on exploring the function and regulation of a particular TE, called mouse endogenous retrovirus type L, MERVL. MERVL is the earliest expressed TE, and is transiently upregulated in mouse embryos at the 2-cell stage. This stage, conserved in human in 4-8 cell embryos, encompasses an essential process called Zygotic Genome Activation, when the embryo begins to turn on its own genes for the first time. These embryos are also considered "totipotent", meaning that they can not only generate embryonic tissues but also extra-embryonic tissues (like placenta). Interestingly, a small proportion of ESCs transiently become "2C-like" in normal culture, also possessing enhanced developmental potency. Here, we will use mouse ESCs and mouse embryos to investigate how and why MERVL regulation is important in early development. Using these tools, we will identify and characterize key factors required to activate and repress MERVL. In turn, we will investigate how these factors regulate the 2-cell stage, and affect ZGA and totipotency. To understand how MERVL and other TEs are directly regulated, we will combine genome-editing systems, called CRISPR/Cas9, with recent biochemical tools to pull out sets of proteins that bind MERVL. Lastly, we will explore the conservation of MERVL function and regulation in human cells, where a similar TE, HERVL, is known to play a conserved role. We aim to a) understand how HERVL regulates the 4-8 cell stage and human ZGA b) investigate how new HERVL regulators might contribute to specific cases of disease. These studies will significantly increase our understanding of how TEs contribute to early development, and will shed insight on how such processes are perturbed in disease.