p21 is a cyclin-dependent kinase inhibitor.
P21 or P-21 may also refer to:
Proteins p21/ras, part of the RAt Sarcoma family of proteins Transforming protein p21, encoded by the HRAS gene in humans
Vessels , a minelayer and patrol ship of the Argentine Navy , a submarine of the Royal Navy , a patrol vessel of the Irish Navy , of the Armed Forces of Malta
Other uses Curtiss XP-21, an American experimental fighter aircraft P21 road (Ukraine) Papyrus 21, a biblical manuscript Yao language Partnership for 21st Century Skills; see 21st century skills Platform 21, stylised as p21 Prophet 21, Distribution Software for businesses published by Epicor P21, three-dimensional space group number 4