P21 (CDKN1A / WAF1 / CIP1 / SDI1)

What it is

P21 (CDKN1A/WAF1/CIP1/SDI1) is a small, intrinsically disordered protein and broad-specificity inhibitor of cyclin-dependent kinases (CDKs). It is a primary transcriptional target of the tumor suppressor p53, induced in response to DNA damage, oxidative stress, irradiation, and other cellular stressors, leading to cell cycle arrest in G1 and G2 phases. P21 also inhibits the activity of proliferating cell nuclear antigen (PCNA), independently of CDK inhibition, modulating DNA replication speed and origin firing during S phase. Beyond cell cycle arrest, p21 participates in apoptosis regulation, transcriptional regulation, cellular senescence, autophagy, and DNA repair. Its subcellular localization critically determines its function: nuclear p21 is associated with anti-tumorigenic (CDK-inhibitory, cell cycle arresting) activity, whereas cytoplasmic p21 is associated with pro-survival, anti-apoptotic, and pro-tumorigenic effects — including activation of the NFκB pathway and resistance to chemotherapy. Post-translational modifications, particularly phosphorylation (e.g., by AKT), regulate p21 intracellular localization and stability. P21 is degraded via both ubiquitin-dependent (N-terminal ubiquitinylation) and ubiquitin-independent (20S proteasome via PSMA3 Trapper / RRLIF degron) pathways. Redox-sensitive cysteine residues within p21's Kinase Inhibitory Domain (KID) enable direct sensing of reactive oxygen species (ROS), allowing p21 to directly regulate CDK activity in response to oxidative stress. P21 expression is regulated at the transcriptional level by p53-dependent and -independent mechanisms, and post-transcriptionally through mRNA stability, translation, and protein turnover. Epigenetic silencing via promoter methylation and histone deacetylation/methylation (e.g., by CTIP2/SUV39H1) can suppress p21 transcription. The term 'P21' also refers to RAS proto-oncogene proteins (HRAS, KRAS, NRAS), which are structurally and functionally unrelated GTPases; this record concerns exclusively the CDK inhibitor CDKN1A/p21WAF1 unless otherwise noted.

Class: Cyclin-Dependent Kinase (CDK) Inhibitor — CIP/KIP family; intrinsically disordered protein (IDP)

What it's studied for

• Cell cycle regulation and tumor suppression (general) Mixed
  • P21 induces G1 arrest and blocks S phase by inactivating CDKs and inhibiting PCNA; overexpression suppresses tumor growth in vitro and in vivo. PMID 8931660 Gartel AL et al., Proc Soc Exp Biol Med (1996)
  • Review: p21 promotes cell cycle arrest in response to diverse stimuli; functions in apoptosis, transcriptional regulation, and DNA repair depend on subcellular localization and protein interactions. PMID 27156098 Karimian A et al., DNA Repair (2016)
  • Review: nuclear p21 is anti-tumorigenic; cytoplasmic p21 is pro-tumorigenic; subcellular localization is controlled by post-translational modifications including phosphorylation. PMID 42201575 Maheshwari M & Harioudh MK, Mol Biol Rep (2026)
• Cancer radiotherapy response modulation Mixed
  • Review: p21 regulates radiation responses via cell cycle arrest, apoptosis, DNA repair, senescence, and autophagy; proposed as a biomarker and therapeutic target for radiotherapy. PMID 33547489 Kuang Y et al., J Cancer Res Clin Oncol (2021)
  • All esophageal SCC cell lines increased p21 protein after irradiation (3–24 Gy); p21 induction was p53-independent in mutation-bearing lines. PMID 9698513 Rigberg DA et al., J Surg Res (1998)
  • p21-deficient cancer and normal cells showed higher γH2AX induction in S phase in response to Wee1 inhibitor MK1775; p21 deficiency sensitized to combined MK1775 + CHK1 inhibitor and to ionizing radiation. PMID 30943845 Hauge S et al., Cell Cycle (2019)
• Bladder cancer — intravesical tumor suppressor replacement therapy (mRNA-LNP) Animal studies only
  • Intravesical p21 mRNA-LNP suppressed tumor growth, restored p21 expression, and preserved urothelial architecture without obvious adverse effects in an orthotopic mouse bladder cancer model; in vitro p21 restoration suppressed proliferation, viability, and clonogenicity of bladder cancer cells. PMID 42144924 Zeng J et al., FASEB J (2026)
• Bladder cancer — BCG response modulation In vitro only
  • p21 overexpression caused G0/G1 arrest and decreased basal caspase-3 in T24 urothelial carcinoma cells; p21 expression was required for BCG's direct cytotoxic effect but not necessary for BCG-induced cell cycle arrest. PMID 19450997 See WA et al., Urol Oncol (2010)
• Osteoarthritis / cartilage repair Mixed
  • P21 expression was reduced in human OA cartilage; P21 overexpression enhanced chondrocyte anabolism and reduced oxidative stress; DHA@HAMA nanoparticles restored P21 via PI3K/AKT pathway, mitigating cartilage erosion in mouse models. PMID 42192449 Fang C et al., J Nanobiotechnol (2026)
  • Lentiviral shRNA achieving ~80% p21 mRNA reduction significantly improved cartilage repair in full-thickness defect mouse models; 90% transduction efficiency in chondrocytes; p21 expression negatively correlated with repair outcomes. PMID 42007882 Larijani L et al., FASEB J (2026)
  • p21−/− iPSCs engineered with iCasp9 showed similar cartilage regeneration to wild-type iPSCs; iCasp9 activation (AP20187) eliminated transplanted iPSCs and prevented tumor formation without ablating cartilage repair. PMID 41259786 Larijani L et al., Stem Cells Transl Med (2025)
• Melanoma prognosis and chemotherapy resistance Human observational
  • High p21 gene/protein expression predicted poor prognosis in human melanoma; cytoplasmic p21 was higher in metastatic versus primary melanoma; p21 overexpression increased dacarbazine resistance; AKT/PI3K inhibition reduced p21 phosphorylation and cytoplasmic translocation. PMID 41462552 Colaneri GN et al., Sci Rep (2025)
• Pancreatic endocrine tumor — prediction of liver metastases (biomarker) Human observational
  • High p21 IHC score (Allred ≥4) had 93% sensitivity, 68% specificity, and 94% NPV for liver metastases in pancreatic endocrine tumors; p21 outperformed conventional clinico-pathological criteria. PMID 37889062 Nasir A et al., Cancer Genomics Proteomics (2023)
• Ovarian cancer — senescence induction and therapeutic targeting In vitro only
  • PTEN/TRIM39 axis stabilized p21 in SKOV3 ovarian cancer cells, promoting senescence; TRIM39 overexpression increased p21 and senescent cell number. PMID 34643308 Ke X et al., Cell Biol Int (2022)
  • High p21 expression found in ~50% of ovarian CCC patients; p21 inhibitor UC2288 attenuated proliferation and induced apoptosis in CCC cell lines, more prominently in high-p21 JHOC-9 cells. PMID 32988887 Minagawa Y et al., Anticancer Res (2020)
  • Low p21/WAF1 expression correlated with high tumor grade, advanced stage, and poor overall survival in 316 epithelial ovarian cancers; p21-/p53+ phenotype conferred highest risk of recurrence. PMID 10206307 Anttila MA et al., Br J Cancer (1999)
• Breast cancer — prognosis and therapy response Human observational
  • Review: nuclear p21 acts as tumor suppressor in breast cancer; cytoplasmic p21 has anti-apoptotic/oncogenic roles; subcellular localization affects chemotherapy responsiveness and prognosis. PMID 30255294 Zohny SF et al., Breast Cancer (2019)
  • Nuclear p21 detected in 82.4% of primary breast tumors; highly p21-positive patients had significantly improved overall survival; p21+/p53- associated with good outcomes, p21-/p53+ with poor outcomes. PMID 10398154 McClelland RA et al., J Pathol (1999)
  • USP30-AS1 lncRNA suppresses p21 via HnRNPF (cytoplasmic, mRNA destabilization) and EZH2/c-Myc (nuclear, transcriptional repression) axes; USP30-AS1 knockdown upregulated p21 and suppressed breast cancer cell proliferation. PMID 41492473 Jiang Y et al., Genes Dis (2026)
• Hepatocellular carcinoma — lincRNA-p21 as tumor suppressor Animal studies only
  • LincRNA-p21 was downregulated in hepatocellular carcinoma tissues; low expression correlated with high disease stage and poor survival; overexpression inhibited tumor growth in xenograft models and potentiated sorafenib response via ER stress induction. PMID 26305675 Yang N et al., Oncotarget (2015)
• Liver fibrosis — lincRNA-p21 as serum biomarker in chronic hepatitis B Human observational
  • Serum lincRNA-p21 levels were lower in 417 CHB patients than 363 healthy controls; negatively correlated with liver fibrosis stage; AUC 0.854 with 100% sensitivity, 70% specificity for discriminating fibrosis from controls. PMID 28107589 Yu F et al., J Viral Hepat (2017)
• Non-small cell lung cancer (NSCLC) — lincRNA-p21 and prognosis/angiogenesis Human observational
  • High lincRNA-p21 associated with poor cancer-specific survival in NSCLC adenocarcinoma patients; lincRNA-p21 inhibition reduced angiogenesis-related gene expression and VEGF-A secretion in vitro; high lincRNA-p21 tumors had higher microvascular density. PMID 27496652 Castellano JJ et al., J Thorac Oncol (2016)
• Esophageal squamous cell carcinoma — lincRNA-p21 as tumor suppressor In vitro only
  • LincRNA-p21 knockdown increased viability, growth, and migration of ESCC cells; lincRNA-p21 negatively regulated CDH5 via YTHDC1 RNA-binding protein. PMID 37766459 Wang J et al., Acta Biochim Biophys Sin (2023)
• Cellular senescence monitoring and elimination (transgenic mouse models) Animal studies only
  • p21-ATD mouse model enabled in vivo tracing and elimination of p21-expressing senescent cells; elimination of p21+ hepatocytes promoted engraftment of grafted hepatocytes and liver repopulation in FAH−/− injury model. PMID 38582962 Chen M et al., Mol Ther (2024)
  • p21-3MR mouse model allowed monitoring and elimination of p21-high cells in vivo; clearance of p21-high cells improved doxorubicin-induced multi-organ toxicity. PMID 36982640 Yi Z et al., Int J Mol Sci (2023)
• Diabetes and metabolic disorders Mixed
  • Review: p21 is linked to diabetes pathogenesis, pancreatic function, glucose homeostasis, and insulin resistance; therapies targeting p21 to alleviate metabolic disorders are discussed, though clinical data are limited. PMID 39684919 Elmitwalli O et al., Int J Mol Sci (2024)
• Sepsis / LPS-induced cardiac dysfunction Animal studies only
  • p21 global knockout mice showed aggravated LPS-induced cardiac dysfunction (5 mg/kg LPS i.p.); p21 modulated inflammatory/oxidative stress and autophagy (via LC3B interaction) in mouse hearts. PMID 32694519 Huang S et al., Cell Death Dis (2020)
• Systemic lupus erythematosus (SLE) — genetic association Human observational
  • p21 SNP alleles (p21US A and p21-1022 A) were significantly associated with SLE in a case-control study of 516 patients and 693 controls; p21US A was associated with arthritis in SLE patients; differential p21 mRNA expression observed by genotype. PMID 16837471 Kong EK et al., Rheumatology (2007)
• Chronic lung inflammation Animal studies only
  • p21 knockout mice subjected to repetitive LPS inhalation showed reduced senescent cells, alleviated chronic bronchitis manifestations, and improved fitness; epithelial and endothelial cells (not immune cells) mediated p21-dependent inflammatory response. PMID 36996500 Levi N et al., Aging (2023)
• Neovascularization / endothelial progenitor cell regulation Animal studies only
  • p21−/− mice showed increased mature EC proliferation and EPC numbers but sensitized ECs to apoptosis; p21+/− mice (one-third p21 protein levels) showed increased neovascularization without apoptosis sensitization; complete p21 deficiency did not increase neovascularization despite increased proliferation. PMID 14752032 Brühl T et al., Circ Res (2004)
• Osteoblast differentiation and bone repair Animal studies only
  • p21 null osteoblasts differentiated faster, were more susceptible to BMP-2-induced differentiation, and underwent increased differentiation-related apoptosis; transient p21 overexpression delayed differentiation onset in wild-type and null osteoblasts. PMID 12733720 Bellosta P et al., J Bone Miner Res (2003)
  • Loss of p21 increased trabecular bone formation post-injury; loss of E2f1 (downstream of p21) increased cortical bone but reduced osteoblast/osteoclast/chondrocyte numbers and impaired repair; double-knockout abrogated pro-osteogenic effects of p21 loss. PMID 38643416 Premnath P et al., Calcif Tissue Int (2024)
• Autophagy regulation in cancer In vitro only
  • p21 negatively regulates basal autophagy at physiological concentrations via AKT-driven ROS accumulation; p21 has oncogenic activity during early tumor development while its inhibition favors established tumor cell survival. PMID 36522339 Maheshwari M et al., Cell Death Dis (2022)
• CNS aging and exercise — context-dependent p21 signaling Animal studies only
  • Review: physiological exercise attenuates persistent p21-associated signatures in hippocampus/cortex in aging/metabolic distress models; acute exercise transiently increases hippocampal Cdkn1a; excessive exercise produces sustained hippocampal p53-p21 activation with oxidative injury, apoptosis, and cognitive impairmen PMID 42632465 Sun C et al., Ageing Res Rev (2026)
• Melanogenesis regulation In vitro only
  • CRISPR/Cas9-mediated p21 knockout in B16F1 cells reduced SA-β-galactosidase activity, melanin synthesis, collagen production, and expression of MITF, TRP-1, TRP-2, TYR, CDK2, Cyclin D, Cyclin E, and p-ERK, indicating p21 is required for MITF-driven melanogenic signaling. PMID 42214793 Kim GH & Kim MM, J Biotechnol (2026)
• Macrophage activation / inflammatory regulation Animal studies only
  • Review: p21 is a negative regulator of macrophage activation, inhibiting LPS-dependent induction of TNF-α and IL-1β via NF-κB activity modulation; p21−/− mice showed increased susceptibility to septic shock. PMID 19283709 Lloberas J & Celada A, Eur J Immunol (2009)
• RAS-p21 oncogenic protein targeting (distinct protein — for disambiguation) Human observational
  • Review of peptide and small-molecule approaches targeting oncogenic RAS-p21 (GTPase, unrelated to CDKN1A). AMG510 (sotorasib) and MRTX849 (adagrasib) are described as FDA-approved for RAS-p21-induced NSCLC and colorectal carcinomas. NOTE: RAS-p21 is a distinct protein from CDK inhibitor p21/CDKN1A. PMID 36831007 Pincus MR et al., Biomedicines (2023)

Community-reported dosing

Tier key: Research = PMID-cited study · [C] = scraped community source · [S] = model-synthesized from aggregate community reports (softer evidence). How we source.