Abstract JTX-2011, an ICOS agonist antibody, is the first clinical program to emerge from Jounce's Translational Science Platform, which couples the choice of target mechanism to potential predictive biomarkers of response. ICOS (Inducible T cell CO-Stimulator), a co-stimulatory molecule expressed primarily on T lymphocytes, was prioritized as a target based on preclinical and clinical data suggesting that it plays an important role in the immune response to cancer. Fundamental data from our preclinical studies shows that tumor reduction occurs only in animals when a certain percentage of ICOS positive immune cells are resident within the tumor or in combination with a PD-1 inhibitor. Thus ICOS expression is a key element of our biomarker-driven approach in the ICONIC clinical trial. Based on this biomarker approach, we have identified gastric cancer as a cancer of potential interest for an ICOS-targeted immunotherapy approach. Gastric adenocarcinoma was identified as a tumor of interest based on the integrated analysis of RNA, DNA and clinical data from the Cancer Genome Atlas (TCGA). This analysis was performed within stomach adenocarcinoma (STAD) to understand the context in which ICOS is expressed. ICOS levels were correlated to gene signatures of immune infiltrate as well as other clinical attributes and molecular markers. ICOS and PD-L1 levels were assessed by IHC in human tumor samples and in biopsies from ICONIC participants.IHC and RNA analyses reveals a dynamic range of ICOS expression across gastric adenocarcinoma tumors, with high prevalence in both EBV+ and MSI-H tumors as well as a subset of EBV-/MSS tumors. While there is a correlation between ICOS, ICOS signature, PD-L1 and IFNγ signatures, RNA analysis indicates that a subpopulation of gastric tumors with lower levels of PD-L1 expression may be ICOS positive. Integrative analysis of tumors identifies gastric cancer as an attractive indication for exploration of JTX-2011 plus a PD-1 inhibitor based on the relatively high frequency of ICOS expression within this tumor type. Since ICOS expression has a dynamic range of expression, an ICOS IHC biomarker is being used to enrich for patients in a gastric cancer cohort, as well as other tumor-specific cohorts in the Phase 2 portion of the ICONIC clinical study. Citation Format: Heather A. Hirsch, Jason Reeves, Tong Zi, Alexander Needham, Edward Stack, David Lee, Emma Lees, Deborah A. Law, Elizabeth Trehu. Genomics based studies of gastric tumors identify ICOS as potential target for therapeutic intervention [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1685.