ErbB2, also known as HER2 (Human Epidermal Growth Factor Receptor-2), is a unique member of the ErbB family of receptor tyrosine kinases [1]. Unlike other members, no ligand has been identified for ErbB2. At normal expression levels, it acts as a shared coreceptor for the ErbB family, and its heterodimeric complexes are activated in response to partner ligands. When overexpressed, kinase activation occurs due to constitutive homodimerization even without a ligand. ErbB2 is involved in important biological processes and is also associated with cancer pathogenesis [1,2].

One of the first consistent genetic alterations in human breast tumors is the amplification of the erbB-2 gene leading to overexpression of the protein [3]. In multiple myeloma patients, upregulated expression of ERBB2 mRNA in malignant plasma cells is correlated with amplified expression of mRNAs for transcription factors that recognize the ERBB2 gene promoter sites, and is associated with increased cancer mortality, shorter progression-free survival, and worse overall survival [4].

In conclusion, ErbB2 plays a crucial role in normal biological processes as a coreceptor in the ErbB family. However, its overexpression is strongly associated with breast cancer and multiple myeloma, highlighting its significance in cancer pathogenesis. Understanding ErbB2 through in vivo studies, such as potentially using KO/CKO mouse models in future research, could further elucidate its functions and provide more insights into cancer treatment strategies.