Human Epidermal growth factor Receptor 2 (HER2) is also called ERBB2, HER-2, HER-2 /neu, NEU, NGL, TKR1 and c-erb B2, and is a protein giving higher aggressiveness in breast cancers. It is a member of the ErbB protein family, more commonly known as the epidermal growth factor receptor family. HER2 is a cell membrane surface-bound receptor tyrosine kinase and is normally involved in the signal transduction pathways leading to cell growth and differentiation. Human ErbB2 consists of 1255 amino acids (aa) including a 22 aa signal sequence, a 630 aa extracellular domain (ECD), a 23 aa transmembrane region, and a 580 aa cytoplasmic domain. HER2 plays a key role in development, cell proliferation and differentiation. HER2 appears to play roles in development, cancer, communication at the neuromuscular junction and regulation of cell growth and differentiation. Phosphoinositide 3-kinase has been shown to play a role in ErbB2 signal transduction. The cytoplasmic domain of ErbB2 has been shown to associate with beta-catenin and plakoglobin. HER2 appears to play roles in development, cancer, communication at the neuromuscular junction and regulation of cell growth and differentiation.