Heregulin1-Beta1(HRG1-Beta1) is one of the isoforms encoded by Neuregulin (NRG) genes. NRGs are synthesized as large transmembrane precursor proteins, and the NRG family has 4 members and 26 isoforms. These isoforms provide large diversities, including different tissue distribution, variable potencies, and different biological functions. HRG1-β1 belongs to Type I HRG1, and is expressed in neural tissue, respiratory epithelia, and heart. In vivo, HRG1 binds and activates both ErbB3 and ErbB4, the transmembrane receptor tyrosine kinase, and is involved in the proliferation, differentiation, and survival of cells. Aberrantly produced HRG1 could be used in the constitute activation of the ErbB receptors; therefore, the upregulation of HRG1 contributes to the development of tumors, including breast cancer.