It has been observed that radiation can induce significantly elevated levels of tumour hypoxia inducible factor-1 (HIF-1), which further increases the proportion of hypoxic cells in tumours to regulate tumour radiosensitivity.143,144 Importantly, HIF-1 has also been confirmed to be an important mediator in hypoxia, maintaining the stem cell phenotype of CSCs.145 It has been reported that under hypoxic conditions, CSCs sorted from FaDu cells, an oropharyngeal cancer cell line, express HIF-1α faster than non-CSCs after radiation. Here, HIF1A is linked to oropharynx cancer.