Under certain conditions, when the cells are blocked in G2 or G2/M, apoptotic or senescent mechanisms are triggered, as demonstrated by cytofluorimetric and densitometric analyses in the present work, leading to the degradation of p21: this occurs especially in prolonged treatments of cancer cells with DNA damaging agents that can determine a p21 inactivation [118,119]. The gene discussed is CDKN1A; the disease is cancer.