HER3 protein, the full name of which is Human Epidermal Growth Factor Receptor 3, is a member of the receptor tyrosine kinase (RTK) family and is involved in biological processes such as cell growth, proliferation, and differentiation. The HER3 protein is encoded by the ERBB3 gene and is part of the EGFR family, which includes EGFR (HER1), HER2, and HER4. Unlike other members of the EGFR family, HER3 protein has weak kinase activity itself and usually needs to form a dimer with other members to activate downstream signaling pathways. HER3 protein plays an important role in various cancers, including breast cancer, ovarian cancer, gastric cancer, and colorectal cancer. It is considered an important target for cancer treatment because overexpression or abnormal activation of HER3 is associated with tumor development and treatment resistance. Antibodies and inhibitors targeting HER3 have been developed and have shown some therapeutic effects in clinical trials. Overall, HER3 protein is an important cell signaling molecule with potential importance in cancer research and treatment.