ABSTRACT Metastatic castration-resistant prostate cancer (mCRPC) is an incurable disease that is expected to account for approximately 33,000 deaths each year in the United States. Therapeutic options are limited for mCRPC patients that extend life. There is an urgent need for developing novel targeted therapies, especially personalized therapies based on genomic alterations in tumors. Recent genomic studies have revealed a variety of actionable molecular targets with underlying genomic alterations. Notably, alterations in genes involved in DNA damage response are among the most common genetic events and enriched in mCRPC. These alterations have been correlated with particular therapeutic vulnerabilities in prostate cancer (PCa) cells. Specifically, defects in homologous recombination (HR) repair would predict sensitivity to inhibition of Poly (ADP-ribose) polymerase (PARP). PARP inhibitors (PARPis) are a new type of targeted therapy, which works by preventing the enzyme PARP from repairing damaged DNA in tumor cells. The BRCA1 and BRCA2 genes encode proteins essential for HR repair. Cancer cells lacking BRCA1/2 depend instead on PARP-regulated DNA repair and are highly sensitive to PARPis. The U.S. FDA has approved two PARP inhibitors (olaparib and rucaparib) for the treatment of mCRPC patients with deleterious BRCA1/2 or HR repair gene mutations. One of the major barriers to effective treatment using PARPis is how to select patients who most likely benefit from PARP inhibition. Resistance to PARPis also represents a formidable clinical problem. Through genome-wide CRISPR (clustered regularly interspaced short palindromic repeats) screens, we recently discovered a number of genes that mediate cellular response and resistance to PARP inhibition. Unexpectedly, we found that loss of CHEK2 (Checkpoint kinase 2), a cell cycle checkpoint regulator and tumor suppressor gene, significantly increased PCa cell resistance (instead of sensitivity) to PARP inhibition. CHEK2 plays a role in HR repair and CHEK2 alterations are currently used to predict olaparib response in the FDA-approved HR repair gene panel. The goal of this project is to determine to what extent loss of CHEK2 renders PCa cells resistant to PARP inhibition and define the underlying mechanisms in preclinical PCa models. At completion of this project, we expect to demonstrate that CHEK2 loss is a biomarker to predict PARPi resistance. The findings from this study may change patient eligibility for PARP inhibition and have a positive impact on future clinical practice.