p21 is a cyclin-dependent kinase inhibitor.

P21 or P-21 may also refer to:

• p21/ras, part of the RAt Sarcoma family of proteins
• Transforming protein p21, encoded by the HRAS gene in humans

• ARA King (P-21), a minelayer and patrol ship of the Argentine Navy
• HMS Oberon (P21), a submarine of the Royal Navy
• LÉ Emer (P21), a patrol vessel of the Irish Navy
• Maltese patrol boat P21, of the Armed Forces of Malta

• 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
• Prophet 21, Distribution Software for businesses published by Epicor

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