The reciprocal regulation of HIF1α and HIF2α was observed in primary breast cancer cells and in MDA-MB-231 cells after 72 h of Jagged1-stimulation, as well as in Notch-activated medulloblastoma D324 cells when cells were treated with CoCl2, a frequently utilized hypoxia-mimetic. The gene discussed is HIF1A; the disease is breast cancer.