Background: The intracellular DNA sensor stimulator of interferon genes (STING) plays a vital role in anti-tumor immune responses by recognition of self-DNA from tumors and by-products of genomic instability. Activation of STING was reported to enhance cetuximab mediated natural killer cell activation and dendritic cell maturation. Previous reports suggested that polymorphisms of cGAS-STING can affect innate immune response. Therefore, we hypothesized that genetic variants in the cGAS-STING pathway may predict first-line treatment outcome in mCRC pts treated with bevacizumab/cetuximab-based chemotherapy. Methods: Genomic DNA from blood samples of pts enrolled in two independent randomized trials, FIRE-3 (cetuximab arm, n = 129; bevacizumab arm, n = 107) and TRIBE (bevacizumab arm, n = 215), was genotyped through the OncoArray, a customized array manufactured by Illumina including approximately 530K SNP markers. The impact on outcome of 7 selected SNPs in 3 genes involved in the STING pathway (cGAS, STING, IFNB1) was analyzed. Results: In the Cetuximab cohort, pts with STING rs1131769 any T allele (N = 29) showed significantly shorter overall survival (36.3 vs 56.07 months) compared to carriers of C/C (N = 95) in both univariate (hazard ratio [HR] = 2.08; 95% confidence interval [CI]: 1.06-4.07; p = 0.003) and multivariate (HR = 2.98; 95%CI 1.35-6.6; p = 0.00848) analysis; Pts carrying IFNB1 rs1051922 any A allele (N = 68) showed significant shorter progression-free survival (10.23 vs 14.1 months) than carriers of G/G (n = 59) in both univariate (HR = 1.87; 95%CI 1.26-2.78; p = 0.00163) and multivariate (HR = 2.03; 95%CI 1.25-3.3; p = 0.004) analysis. No association were observed in the bevacizumab cohort of TRIBE and FIRE-3. Conclusions: Our study demonstrates for the first time that STING and IFNB1 polymorphisms could predict outcomes of Cetuximab-based treatment in mCRC patients; These finding may provide insight for the combination of STING agonist and anti-EGFR treatment in mCRC patients.