We conducted a clinical trial evaluating safety and efficacy of T-mab plus N-mab with S-1/Capecitabine plus oxaliplatin (UMIN000034222). The aim of this study was to investigate the clinical significance of ERBB2 amplification (amp.) and TMB-H for this new combination therapy....Eleven patients with ERBB2 copy number ≥ 50 or TMB-H showed significantly longer PFS (mPFS: 20.8 vs 5.8 months, HR = 0.28 [95%CI 0.09-0.88], p = 0.021) and OS (mOS: not reached vs 15.0 months, HR = 0.10 [95%CI 0.01-0.78], p = 0.007)...High ERBB2 copy number and TMB-H may be a candidate biomarker for this new combination therapy including T-mab and N-mab in patients with HER2-positive advanced gastric cancer.