CDKN1A and head and neck squamous cell carcinoma: To assess whether the lack of radiation-induced senescence seen in null and mutp53 HNSCC cells, relative to wtp53 HNSCC cells, was due to the loss of p53's transcriptional activity in the induction of p21, a protein well known to have an important role in mediating premature senescence, we measured the expression of the p21 gene by qRT-PCR after exposure to 4 Gy.