ABSTRACT Triple negative breast cancer (TNBC) has the poorest clinical outcome amongst all breast cancer subtypes. TNBC lacks the expression of the three major receptors found in other subtypes (estrogen receptor [ER], progesterone receptor, and/or hormone epidermal growth factor receptor-2) making this cancer particularly challenging with regards to treatment modalities. Novel targeted treatments that could kill TNBC cells or sensitize them to chemo- and radiation therapies are highly coveted to increase survival of these patients. Due to its fast proliferative rate, TNBC relies heavily on DNA repair mechanisms for its survival and proteins involved in this important checkpoint are attractive targets for cancer treatment. CAPER (Rbm39) protein was recently shown to be overexpressed in breast cancer specimens compared to normal breast tissues. While CAPER knockdown inhibits breast cancer cell growth, its role on DNA damage and repair mechanisms in breast cancer and its role in TNBC progression and response to chemo- and radiation therapies remain completely unexplored. Our preliminary data demonstrate that knockdown of CAPER expression in TNBC cells increases DNA damage as reflected by increased phosphorylation of H2AX and ATM. The decreased total cell number and increased caspase-3/7 cleavage observed following CAPER knockdown in MDA-MB-231 and BT549 TNBC cells is suggestive of insurmountable DNA damage leading to programmed cell death (apoptosis). The effect of CAPER knockdown on DNA damage in TNBC cells was cell cycle-independent and selective to cancer cells, as non- tumorigenic cells lack the expression of CAPER and remain unaffected following delivery of lentiviral CAPER shRNAs. Our preliminary results also revealed that DNA repair proteins RAD-51, C-Alb and RB were significantly downregulated in TNBC following CAPER knockdown. We posit that CAPER overexpression in TNBC plays an important role in protection against DNA damage by optimizing DNA repair pathways. The current proposal builds on our previous work and preliminary results and aims to delineate the roles of CAPER in basal DNA damage/repair pathways and to determine its clinical relevance in TNBC growth using both immunocompromised (xenograft) and immunocompetent (syngeneic) orthotopic mouse models (Aim 1). Importantly, we will also use these TNBC cells and mouse models to test the unexplored role of CAPER in the response to DNA damaging chemo- and radiation therapies in both in vitro and in vivo settings (Aim 2). The current proposal will validate the role of CAPER as an important signaling molecule in the progression of TNBC as well as response to DNA damage using clinically relevant models that also incorporate immune surveillance and will be insightful in the further development of targeted therapies for the treatment of TNBC.