Illustration of the HIF-1a (hypoxia-inducible factor-1a, red) ARNT (aryl hydrocarbon receptor nuclear translocator, yellow) complex (greyish). This complex binds to DNA (deoxyribonucleic acid) at specific hypoxia response elements HRE (blue). Cells respond to reduced oxygen levels through the activation of a particular set of genes, including those involved in energy metabolism and apoptosis, as well as genes whose gene products are related to increased oxygen delivery or to facilitate metabolic adaptation to hypoxia. HIF-1 thus plays an essential role in tumour angiogenesis and pathophysiology of ischemic disease. miRNAs (micro ribonucleic acids) regulate the HIF switch during hypoxia.