Background: Somatic mut in DDR genes have been associated with increased sensitivity to cisplatin in UC. Higher mut load has correlated with response to immune checkpoint blockade. We hypothesized that DDR mut result in higher mut load and DDR mut UC may be particularly sensitive to both chemotherapy and immune checkpoint blockade. Methods: Three cohorts were utilized: (1) TCGA UC cohort (n = 389), (2) Mount Sinai (MS) cohort (n = 67) of UC (cystectomy) specimens subjected to targeted exome sequencing for 341 genes (MSK-IMPACT), and (3) Phase 2 trial of gemcitabine, cisplatin, plus ipilimumab (GCI) in metastatic UC from which 28/36 enrolled patients (pts) had specimens suitable for whole exome sequencing. DDR mut were defined as somatic alterations in one of 52 genes. Deleterious (del) mut were defined as nonsense, frameshift, splice site, or hotspot point mut. Results: The mut load using all genes in the TCGA cohort, and restricted to the 341 IMPACT genes, were highly correlated (r_s= 0.81, p < 0.001). Associations between del DDR mut and mut load are shown (Table). In the MS cohort, CD8+cells/mm2 by IHC were higher in tumors with del DDR mut versus no DDR mut (p = 0.04). In the GCI cohort, the sensitivity, specificity, positive predictive value, and negative predictive value of a del DDR mut for objective response to treatment was 40.9% (95% CI 20.7-63.7%), 86.7% (95% CI 42.1-99.4%), 90% (55.5-99.8%), and 31.6% (95% CI 12.6-56.6%), respectively. Median progression-free survival in the GCI cohort was 308 days (95% CI 270-NR) in del DDR mut and 196 days (95% CI 185-372) in others (p = 0.24). Notably, 2/9 pts with del DDR mutations, and with the highest mut loads, achieved complete responses after GCI and are alive without evidence of disease at 2.1+ and 1.8+ years. Conclusions: DDR mut are associated with higher mut load in UC, a high likelihood of response to GCI, and may identify a subset of pts achieving durable disease control. GC plus PD-1/PD-L1 blockade should be explored in DDR mut UC. Clinical trial information: NCT01524991