Recent reports have shown that there is a strong correlation between p21, ROS and senescence with results varying by stimulus and cell type.27, 28, 29 As our mode of treatment for these HNSCC cells was ionizing radiation, we furthered our investigation into the p21–senescence relationship by measuring the effect that p21 overexpression would have on ROS. The gene discussed is CDKN1A; the disease is head and neck squamous cell carcinoma.