In this study, we have shown that HER2 overexpression in MCF7 cells leads to an increase in HIF-2α levels in normoxia and the increased upregulation of HIF-2α in response to hypoxia; this demonstrates novel mechanistic differences in the growth factor-mediated regulation of HIF-1α and HIF-2α in breast cancer cells. The gene discussed is ERBB2; the disease is breast carcinoma.