To understand whether HRG-β1 can induce HER2 activation via heterodimerization with HER3 in human breast cancer cell line MCF-7, we analyzed the status of tyrosine phosphorylation of HER2 and HER3, a characteristic for RTK activation, in response to HRG-β1 in MCF-7 cells. The gene discussed is ERBB3; the disease is breast carcinoma.