A protein with a molecular mass of roughly 21 000 kDa that plays a crucial role in regulating the cell cycle and enabling repair of damaged DNA before the cell proceeds to replication and division. Expression of the p21 gene is regulated by the product of the tumour-suppressor gene p53, and occurs in response to DNA damage. The p21 protein binds to the cyclin-dependent kinase (CDK) present during the G₁ and S phases, thus inhibiting its activation by cyclin. This pauses the cell cycle until the DNA is repaired, whereupon the p21 breaks down and the cell cycle resumes.