Because HIF signaling mediates increased fibulin expression under hypoxic conditions [18] and because fibulins seem to play an instrumental role in breast cancer chemoresistance [19], we further investigated whether hypoxia-induced EFEMP1 influences the tumorigenic properties of CSCs as a both key regulator of hypoxia and a downstream effector of hypoxia-induced HIF2α. This evidence concerns the gene EPAS1 and breast cancer.