Receptor tyrosine-protein kinase erbB-2, a 185 kDa protein, also known as CD340 (cluster of differentiation 340), proto-oncogene Neu, Erbb2 (rodent), or ERBB2 (human) is a protein that in humans is encoded by the ERBB2 gene. The ERBB2 gene is also frequently called HER2 (from human epidermal growth factor receptor 2) or HER2/neu.

HER2 is a member of the epidermal growth factor receptor (EGFR/ERBB) family. HER2 has been shown to play an important role cancer development, metastasis and chemotherapeutic resistance in many kinds of multiple cancers including, breast cancer, lung cancer, gastric cancer, ovarian cancer and so on through its overexpression or mutation. HER2 is also become an important therapeutic target for cancers.

Tyr1248 and Tyr1221/1222 are the major autophosphorylation sites on ErbB2, and the phosphorylation on these sites correct to the activity and signaling transduction of HER2.

Phosphorylation at tyrosine 1221/1222 also has been approved to improve the prediction of poor survival for breast cancer patients with hormone receptor-positive tumors. Thus, both HER2 and phosphorylated HER2 are important targets for cancer diagnosis.