ERBB3 and breast cancer: Another example is MCLA 128, an IgG1 BsAb specifically binding HER2 and HER3, which potently inhibits heregulin-driven signaling of HER2/HER3 and downstream PI3K/Akt signaling to suppress tumor cell survival and proliferation.183 In addition to those directed to breast cancer, clinical studies on MCLA 128 are ongoing in patients with metastatic pancreatic ductal adenocarcinoma or NSCLC that harbor fusion neuregulin 1 (NRG1) genes.