The DNA double helix, which stores our genetic information, is constantly damaged by various agents such as reactive chemicals or even sunlight, but our cells are equipped with machineries made up of proteins that repair damages to DNA. An example of damage is a cut in the double helix called a double-strand break. The main mechanism for the repair of double-strand breaks is called homologous recombination (HR). Many proteins contribute to HR DNA repair. Malfunctions in HR have been linked to ovarian cancer and it is therefore important to understand how HR works to be able to later develop new drugs to treat ovarian cancer. In our proposed work, we will use powerful electron microscopy (called cryo-electron microscopy or cryo-EM) to take pictures of two proteins called 53BP1 and USP51 bound to a complex of DNA and histone proteins called the nucleosome. In our cells, DNA is not free. It is always part of nucleosomes. When there is a double-strand break in the DNA, the nucleosome becomes tagged by a chemical (called ubiquitin) that signals that DNA needs to be repaired. 53BP1 and USP51 both recognize this ubiquitin signal and work together as part of the HR repair process which is very regulated and complex. In fact, a natural role of 53BP1 is to prevent the HR activity. By contrast, USP51 stimulates HR activity. USP51 recognizes the ubiquitin as mentioned above, but then acts as a molecular scissor that cleaves ubiquitin from the nucleosome. This removal of ubiquitin activates HR DNA repair, presumably by preventing the HR inhibitory activity of 53BP1. With cryo-EM, we will be able to see in great detail (we almost see atoms) how 53BP1 and USP51 bind to the nucleosome with ubiquitin. This high-resolution view will help us understand how 53BP1 and USP51 function. For example, we will be able to understand how USP51 cleaves ubiquitin in atomic detail. This is very useful information as it could later help scientists develop drugs that prevent USP51 from cleaving ubiquitin. These drugs would favor 53BP1 binding to the nucleosome and would therefore inactivate HR DNA repair. The PARP inhibitor drugs currently used for ovarian cancer treatment specifically kill cancer cells that have defective HR DNA repair. However, patients can become resistant to PARP inhibitors when after a while the HR repair activity becomes less defective in the cancer cells. Inhibitors of USP51 would be expected to restore efficient PARP inhibition and killing of cancer cells.