In this study, we recruited GC patients who received treatment with FOLFOX chemotherapy (oxaliplatin (L-OHP)+calcium folinate (CF)+5-fluorouracil (5-Fu)) and trastuzumab and detected the HER2 copy number status by ddPCR in plasma samples every 2 months during the 12 months of follow-up, to evaluate its potential clinical use as a biomarker for monitoring the tumor status and treatment responses during the treatment period. Here, ERBB2 is linked to neoplasm.