HES1 and breast cancer: Notch-mediated activation of HES1 and HEY1 under hypoxic environment involves factors like HIF-1α and HIF-2α, and it was recently reported that HIF-1α binds to the HES1 promoter directly under hypoxic conditions and that a HIF-1α knock-down lowers both HES1 and HEY1 expression in BC cells [48].