HIF1A and neoplasm: In addition, HIF-1α expression has been shown to be tightly regulated by local oxygen levels, which may be fluctuating in the tumor mass due to cycling hypoxia; moreover, the fluctuation of HIF-1α expression in response to oxygen or other stimuli are peculiarly and primarily regulated at the protein level, as HIF-1α protein stabilization occurs before mRNA transcription [50, 51].