ERBB2 and gastric cancer: Peng et al. and Zhou et al. found that the serum HER2 extracellular domain (ECD) was highly correlated with tissue HER2 status in metastatic gastric cancer; there was also a significant difference in the serum HER2 ECD levels between patients with HER2 IHC 3+ and those with HER2 IHC 2+/FISH+, which supported the clinical utility of serum HER2 ECD detection in patients with advanced gastric cancer [14, 15].