252 Background: Metastatic colorectal cancer (mCRC) patients exhibit intertumoral genetic heterogeneity, making it difficult to apply precision medicine. In the molecular genetic profile of mCRC, there are relatively few other gene alterations that significantly impact treatment decisions beyond KRAS and MSI status. We aim to investigate the prognostic impact of Homologous recombination deficiency (HRD)-related genetic alterations in mCRC patients, focusing on how mutation status influences on patient outcomes. Methods: The analysis included 364 mCRC patients who underwent comprehensive genomic profiling at Korea University Anam Hospital from Sep. 2016 to Dec. 2020. HRD was defined by the presence of pathogenic variants in genes including ATM, ATR, BARD1, BRCA1/2, BRIP1, CDK12, CHEK2, FANCA, MRE11A, PALB2, RAD50/51. Overall survival (OS) and progression-free survival (PFS) were evaluated using the Kaplan-Meier method. Multivariate Cox proportional hazards regression analyses were performed to assess the independent impact of HRD status. Results: Of the total cohort, 181 of patients (49.78%) harbored KRAS mutations. HRD gene mutations were identified in 38 of patients (10.4%) of patients, predominantly in ATM, BRCA 1/2, ARID1A genes. KRAS-HRD co-mutations were present in 18 of patients (4.7%). There is no significant difference in HRD status between the KRAS mutant and wild-type groups (p=0.73). While first-line treatment PFS was independent of KRAS mutational status (wt 10.8 vs. mut 9.6 months, p=0.31), it was significantly associated with HRD gene mutation status (16.2 vs. 10.6 months, p=0.001). OS was similar with KRAS mutations (34.7 vs. 32.9 months, p=0.72) and HRD mutations (34.7 vs. 39.1 months, p=0.73). However, patients with both KRAS and HRD mutations had significantly worse OS compared to those with either KRAS and HRD mutation (21.3 vs. 36.7 vs. 65.3 months, p=0.034, <0.001). A multivariate analysis was performed to evaluate factors affecting OS in mCRC patients. Among the variables, metastasectomy, which 44% of patients underwent, had a significantly low HR (=0.28) and would contributed to more favorable outcomes when evaluating OS. Neither KRAS nor HRD mutation alone was a significant prognostic factor, but their co-mutation was significantly associated with poor survival (HR=2.39, p=0.048). Conclusions: Poor OS in patients with KRAS-HRD co-mutations, despite metastasectomy, highlights the need for additional treatment strategies beyond surgery. Future studies with larger sample sizes and analyses stratified by metastasectomy status are necessary to strengthen these findings. Variables HR (95% CI) p-value Metastasis site Liver 2.16 (1.60 – 2.92) < 0.001 Peritoneum 1.52 (1.01 – 2.11) 0.013 Bone 1.69 (0.96 – 2.97) 0.067 Metastasectomy 0.28 (0.47 – 0.74) < 0.001 KRAS 1.03 (0.77 – 1.37) 0.826 HRD 0.75 (0.40 – 1.41) 0.367 Co-mutation 2.39 (1.01 – 5.66) 0.048