ERBB2 and gastric cancer: Han et al. integrated CD137 (4-1BB) into chA21 CAR-T cells to develop second-generation chA21-4-1BBz CAR-T cells that are strongly selective for cells that overexpress human epidermal growth factor receptor 2 (HER2), such as SKOV3-human ovarian cancer and NCI-N87-human gastric cancer.