Project Summary/Abstract Repair of chromosome breaks is essential for normal cell growth, protects against and promotes cancer- causing genome rearrangements, and determines the effectiveness of many cancer therapies. In recent work our group implicated DNA polymerase theta in a pathway for repairing chromosome breaks that favors short sequence repeats (microhomologies). In Aim 1 we will outline the steps in Theta mediated end joining (TMEJ) – its molecular mechanism - and determine the requirements for each step. We will employ a combination of defined extrachromosomal substrates, analysis of repair of a targeted chromosomal break, and quantitative assessments of effects on cell growth and viability when cells are defective in various steps. In aim 2 we will clarify the relationship between Theta mediated end joining and another pathway for repair of chromosome breaks, homologous recombination. Specifically, we will investigate how defects in BRCA1 and 53BP1, genes involved in early steps common to both TMEJ and homologous recombination, impact the choice between repair by these two pathways. We will then also address the importance of the TMEJ pathway as an alternative to Holliday junction resolution, a late step specific to the homologous recombination pathway. We will exploit assays for chromosomal end structure and repair, as well as genetic tools available through use of the invertebrate model D. melanogaster. In aim 3 we will systematically look for novel genetic interactions with Polymerase theta that are important for both general cell viability and cellular resistance to ionizing radiation. We will employ a lentiviral vector based CRISPR library that was curated to focus on a subset of 310 genes relevant to the DNA damage response. The proposed experiments will speak to how an important but as-yet poorly understood pathway works, and especially when and why it represents the best solution to various problems encountered in the course of repair of chromosome breaks.