ERBB2 and neoplasm: Results from preclinical studies of 4-1BB/HER2 or 4-1BB/PD-L1 bispecific antibodies reflected the aims of this design, restricting T cell stimulation to the tumor environments that are rich in HER2 or PD-L1, respectively, and reducing the toxicity caused when 4-1BB is activated in the periphery, including in the liver (98, 99).