TP53 and head and neck squamous cell carcinoma: P21 represents a major mediator of p53-induced cell cycle arrest in the G0/G1 phase in response to DNA-damaging agents and other cellular stresses.38 Flow cytometry-based cell cycle analysis of bortezomib-treated HNSCC cells showed a strong G2/M arrest of up to 50% in all cell lines and a small increase in the sub-G1 population, indicating apoptotic cells (Figure 5e).