Despite continuing improvements in the management of colorectal cancer (CRC), the disease remains the second most common cause of cancer mortality in the UK. Around 30% of colorectal tumours are located in the rectum making them more challenging to remove surgically. In locally advanced cases of rectal cancer, chemoradiotherapy is given alongside surgery to improve overall outcome. A significant proportion of patients do not respond to chemoradiotherapy however and identifying the reasons for this will improve their management. BCL-3 isa proto-oncogene, upregulated ina subset of colorectal tumours, correlating with lower patient survival. Previous work identified a role for BCL-3 as a survival factor in colorectal cancer but little is known about the role of BCL-3 in response to the DNA damaging agents used in chemoradiotherapy. This work therefore set out to determine whether BCL-3 expression could help tumours resist therapy.Results presented here demonstrate that loss of BCL-3 in CRC cells sensitises them to ionising radiation, increasing persistence of DNA double strand breaks (DSBs) after treatment. Reporter assays and drug treatmentsshowed that loss of BCL-3 reduced the efficiency of homologous recombination (HR), a major pathway for DSB repair. To better understand the role of BCL-3 in HR, BCL-3 was deleted in a CRC cell line by CRISPR-Cas9 genome editing, allowing the importance of BCL-3 expression to be studied in 3D cell culture. BCL-3 knockout cells were used in phosphoproteomics to gain insight into the mechanism of the role of BCL-3 in HR. Results suggest that loss of BCL-3 may be reducing the chromatin remodelling required for HR, identifying a novel function of BCL-3 in colorectal cancer. Importantly, this work provides rationale for targeting BCL-3 to improve response to chemoradiotherapy as well as for the use of BCL-3 as a predictive marker for the effectiveness of this treatment.