Background: NIVO + chemo and NIVO + IPI demonstrated superior overall survival (OS) vs chemo in previously untreated patients with advanced ESCC in CheckMate 648 (NCT03143153), resulting in approvals in many countries. We present first comprehensive exploratory biomarker analysis results from this trial. Methods: Whole exome sequencing (WES) of baseline tumor tissue and matching blood was performed to assess tumor mutational burden (TMB) and select gene alterations. TMB-high tumors were defined as ≥ 199 mutations/exome. RNA sequencing of baseline tumor tissue was used to assess gene expression signatures (GES). GES subgroups were defined by signature score tertiles. Results: In total, 60% (191/321) of patients receiving NIVO + chemo were both WES- and GES-evaluable; 62% (200/325) and 58% (188/325) of patients receiving NIVO + IPI and 59% (190/324) and 59% (192/324) receiving chemo were WES- and GES-evaluable, respectively. In the NIVO + chemo arm, patients with TMB-high tumors had numerically longer median OS (mOS) compared with TMB-low tumors, whereas mOS was similar between TMB subgroups treated with NIVO + IPI, although the number of patients with TMB-high tumors was small (Table). Higher inflammation and lower β-catenin GES scores were associated with improved OS benefit of NIVO + chemo or IPI vs chemo (Table). Lower fibroblast GES scores were associated with improved OS benefit of NIVO + IPI vs chemo (Table). Additional biomarker analyses, including select gene alterations and GES in tumor cell PD-L1 subgroups, will be presented. Conclusions: These results further support the clinical efficacy of NIVO + chemo and NIVO + IPI in 1L ESCC and suggest enhanced OS benefit in multiple biomarker subgroups. The clinical utility of these biomarkers should be validated in future trials. Clinical trial information: NCT03143153.