Interestingly, one preclinical study showed that hypoxia reduces ERα protein levels, but not ESR1 mRNA levels, in HR+ BC cell lines via the proteasome-dependent degradation of ERα and the HIF-1α-mediated repression of ESR1 gene transcription (Figure 1); however, data linking ERα expression to HIF-1α signaling are still controversial [72,73,74]. Here, HIF1A is linked to breast cancer.