HIF1A and breast cancer: Hypoxia-inducible factors (HIFs) are heterodimeric transcriptional activators composed of an O2-regulated HIF-α subunit (HIF-1α, HIF-2α, or HIF-3α) and a constitutively expressed HIF-1β subunit (30, 31) that regulate the expression of thousands of RNAs when BC cells are transferred from high (20%) to low (1%) O2 (32–34).