ERBB2 and gastric cancer: HER4 and phosphorylated HER4 (p-HER4) expression was increased in trastuzumab-resistant cells while activating the downstream PI3K pathway through the HER4-YAP1 axis to promote the transformation of epithelial cells to mesenchymal cells to maintain the invasiveness of HER2-positive gastric cancer and escape the blockade of trastuzumab (88, 89).