Cancer arises when mutations to our DNA alter the genetic information and change the way our cells normally function. DNA in our body witnesses thousands of lesions on a daily basis. Among these lesions are breaks that occur on both strands of a chromosome, known as double stranded breaks (DSBs), which are highly toxic. In fact, cells in our body have evolved special ways to ensure that when DSBs occur, they are repaired faithfully and promptly to avoid errors in the coding sequence. There are three pathways to repair a DSB in mammalian cells. The preferred pathway is homology-directed repair (HDR) that fixes DNA breaks without altering the original sequence and is hence error-free. DSBs can also be repaired by two additional pathways that are error-prone – the classical Non-Homologous End-Joining (NHEJ) and the alternative NHEJ (alt-NHEJ) pathways. The activity of HDR pathway is absent in many breast cancer cells, and evidence suggests its replacement by the highly mutagenic alt-NHEJ pathway. Hence, the main focus of our proposal is to study the poorly characterized alt-NHEJ pathway of repair and establish its role in breast cancer progression. Using high throughput technology, we plan to uncover novel genes in this pathway and characterize the mechanism by which this repair pathway operates. Ultimately, we will assess the de-regulation of its key components in inherited and sporadic breast cancers. This will provide key steps towards revealing specific targets that can guide more favorable and effective breast cancer treatment strategies.