Heregulin (HRG), a combinatorial ligand for the epidermal growth factor receptor family, is expressed in about 30% of breast cancer tumors. HRG induces tumorigenicity and metastasis of breast cancer cells and promotes hormone-independent growth. Although HRG has been studied mostly in the context of the HRG receptor family, accumulating evidence suggests that HRG plays distinctive and causative roles in breast cancer tumorigenesis independent from the HRG receptors, demanding a comprehensive and independent study of HRG as a unique growth factor. This review provides a consolidated view of HRG and its biological role in the development, progression, and maintenance of breast cancer. Further, it provides further evidence that HRG is implicated in breast cancer resistance and targeting HRG may possibly be a beneficial tool to target a subgroup of breast carcinomas.
