Relevant biomarkers for selection of EC and GC patients eligible for ICI include PD-L1 immunohistochemistry, microsatellite instability (MSI), mismatch repair deficiency (MMR), tumor mutational burden (TMB), Epstein–Barr Virus (EBV) positivity, immune gene signatures, overexpression of HER2, and epigenetic alterations [128]. This evidence concerns the gene ERBB2 and gastric cancer.