ABSTRACT DNA double-strand breaks (DSB)s are eliminated primarily through two mechanistically distinct pathways, namely, non-homologous DNA end joining (NHEJ) and homology-directed repair (HDR). Importantly, HDR also functions to repair damaged DNA replication forks and to protect stressed replication forks against attrition mediated by cellular nucleases. The selection of HDR versus NHEJ is subject to intricate regulatory control and is linked to the cell cycle. Most notably, the 53BP1 axis, consisting of the chromatin epigenetic mark reader 53BP1 and associated factors, including DYNLL1 and the hetero-trimeric CTC1-STN1-TEN1 (CST) complex, functions to restrict HDR in G1 phase of the cell cycle to channel DSBs into NHEJ. Our Program Project research teams at the University of Texas Health Science Center at San Antonio, Columbia University, and Dana Farber Cancer Institute focus on delineating the mechanisms that underpin the DNA end resection and RAD51 presynaptic filament assembly steps of HDR, and how these processes are negatively regulated by the 53BP1 axis, and the manner in which this restriction is overcome by HDR protein complexes that harbor the tumor suppressors BRCA1 and BRCA2. The Protein Biochemistry and Enzymology (PBE) Core is directed by a seasoned nucleic acid biochemist/enzymologist and will help our Program Project overcome technical bottlenecks and achieve project objectives by (1) providing state-of- the-art support in protein expression and purification, protein-protein interaction studies, nucleic acid biochemistry, and also enzymatic assays germane for the research goals being pursued; (2) meeting new challenges through consultation with investigators on matters pertaining to our areas of expertise; (3) training students and fellows in various protein biochemistry and enzymological methods and techniques; and (4) fulfilling the protein production needs of the Structural Biology and Biophysics (SBB) Core. 1