ERBB2 and gastric cancer: In patients with HER2-positive gastric cancer receiving trastuzumab, 29.1%-64% of patients developed loss of HER2 expression during treatment (IHC score <3+ and absence of ISH amplification) and/or loss of HER2 overexpression (IHC “down scoring” from 2+/3+ to 0/1+), At the same time, the heterogeneity of HER2 gene expression increased.