Ukr.Biochem.J. 2026; Volume 98, Issue 1, Jan-Feb, pp. 5-19
doi: https://doi.org/10.15407/ubj98.01.005
Cancer stem cells in recurrence and therapy resistance: biological insights and emerging therapeutic strategies
Y. Tamilselvi*, P. Velmurugan, K. Sivasubramanian
Centre for Materials Engineering and Regenerative Medicine,
Bharath Institute of Higher Education and Research, India;
*e-mail: pughazselvi@gmail.com
Received: 12 May 2025; Revised: 29 September 2025;
Accepted: 30 January 2026; Available on-line: 23 February 2026
Cancer stem cells (CSCs), a resilient subset of tumor cells, able to evade immune detection and rapidly proliferate, are responsible for the metastasis, recurrence, and therapeutic resistance observed across various cancers. Recent research has concentrated on understanding the molecular networks that support CSCs immune evasion, self-renewal, and adaptability. Signaling pathways (Wnt, Notch, Hedgehog, JAK-STAT) and surface markers (CD44, CD133, ALDH1) that characterize CSC behaviour are compiled in this review. We highlight the expanding usefulness of omics technologies, such as CRISPR functional genomics, single-cell transcriptomics, and spatial proteomics, in determining vulnerabilities unique to CSCs and guiding tailored treatment plans.
Keywords: cancer stem cells, resistance to treatment, signalling pathways, surface markers, targeted therapy
References:
- Agliano A, Calvo A, Box C. The challenge of targeting cancer stem cells to halt metastasis. Semin Cancer Biol. 2017;44:25-42. PubMed, CrossRef
- Akhtar S, Hourani S, Therachiyil L, Al-Dhfyan A, Agouni A, Zeidan A, Uddin S, Korashy HM. Epigenetic Regulation of Cancer Stem Cells by the Aryl Hydrocarbon Receptor Pathway. Semin Cancer Biol. 2022;83:177-196. PubMed, CrossRef
- Angius A, Scanu AM, Arru C, Muroni MR, Rallo V, Deiana G, Ninniri MC, Carru C, Porcu A, Pira G, Uva P, Cossu-Rocca P, De Miglio MR. Portrait of Cancer Stem Cells on Colorectal Cancer: Molecular Biomarkers, Signaling Pathways and miRNAome. Int J Mol Sci. 2021;22(4):1603. PubMed, PubMedCentral, CrossRef
- Dakal TC, Dhabhai B, Pant A, Moar K, Chaudhary K, Yadav V, Ranga V, Sharma NK, Kumar A, Maurya PK, Maciaczyk J, Schmidt-Wolf IGH, Sharma A. Oncogenes and tumor suppressor genes: functions and roles in cancers. MedComm. 2024;5(6):e582. PubMed, PubMedCentral, CrossRef
- Bisht S, Nigam M, Kunjwal SS, Sergey P, Mishra AP, Sharifi-Rad J. Cancer Stem Cells: From an Insight into the Basics to Recent Advances and Therapeutic Targeting. Stem Cells Int. 2022;2022:9653244. PubMed, PubMedCentral, CrossRef
- Ayob AZ, Ramasamy TS. Cancer stem cells as key drivers of tumour progression. J Biomed Sci. 2018;25(1):20. PubMed, PubMedCentral, CrossRef
- Eid RA, Alaa Edeen M, Shedid EM, Kamal ASS, Warda MM, Mamdouh F, Khedr SA, Soltan MA, Jeon HW, Zaki MSA, Kim B. Targeting Cancer Stem Cells as the Key Driver of Carcinogenesis and Therapeutic Resistance. Int J Mol Sci. 2023;24(2):1786. PubMed, PubMedCentral, CrossRef
- Limonta P, Chiaramonte R, Casati L. Unveiling the Dynamic Interplay between Cancer Stem Cells and the Tumor Microenvironment in Melanoma: Implications for Novel Therapeutic Strategies. Cancers (Basel). 2024;16(16):2861. PubMed, PubMedCentral, CrossRef
- Ganesh K, Massagué J. Targeting metastatic cancer. Nat Med. 2021;27(1):34-44. PubMed, PubMedCentral, CrossRef
- Bekaii-Saab T, El-Rayes B. Identifying and targeting cancer stem cells in the treatment of gastric cancer. Cancer. 2017;123(8):1303-1312. PubMed, PubMedCentral, CrossRef
- Bocci F, Gearhart-Serna L, Boareto M, Ribeiro M, Ben-Jacob E, Devi GR, Levine H, Onuchic JN, Jolly MK. Toward understanding cancer stem cell heterogeneity in the tumor microenvironment. Proc Natl Acad Sci USA. 2019;116(1):148-157. PubMed, PubMedCentral, CrossRef
- Cacho-Díaz B, García-Botello DR, Wegman-Ostrosky T, Reyes-Soto G, Ortiz-Sánchez E, Herrera-Montalvo LA. Tumor microenvironment differences between primary tumor and brain metastases. J Transl Med. 2020;18(1):1. PubMed, PubMedCentral, CrossRef
- Cole AJ, Fayomi AP, Anyaeche VI, Bai S, Buckanovich RJ. An evolving paradigm of cancer stem cell hierarchies: therapeutic implications. Theranostics. 2020;10(7):3083-3098. PubMed, PubMedCentral, CrossRef
- Franco SS, Szczesna K, Iliou MS, Al-Qahtani M, Mobasheri A, Kobolák J, Dinnyés A. In vitro models of cancer stem cells and clinical applications. BMC Cancer. 2016;16(Suppl 2):738. PubMed, PubMedCentral, CrossRef
- Dean L. Tamoxifen Therapy and CYP2D6 Genotype. In: Pratt VM, Scott SA, Pirmohamed M, Esquivel B, Kattman BL , Malheiro AJ. eds. In: Medical Genetics Summaries. Bethesda (MD): National Center for Biotechnology Information (US); 2012. PubMed
- Dong Q, Liu X, Cheng K, Sheng J, Kong J, Liu T. Pre-metastatic Niche Formation in Different Organs Induced by Tumor Extracellular Vesicles. Front Cell Dev Biol. 2021;9:733627. PubMed, PubMedCentral, CrossRef
- Goenka A, Khan F, Verma B, Sinha P, Dmello CC, Jogalekar MP, Gangadaran P, Ahn BC. Tumor microenvironment signaling and therapeutics in cancer progression. Cancer Commun (Lond). 2023;43(5):525-561. PubMed, PubMedCentral, CrossRef
- Eun K, Ham SW, Kim H. Cancer stem cell heterogeneity: origin and new perspectives on CSC targeting. BMB Rep. 2017;50(3):117-125. PubMed, PubMedCentral, CrossRef
- Filipów S, Łaczmański Ł. Blood Circulating miRNAs as Cancer Biomarkers for Diagnosis and Surgical Treatment Response. Front Genet. 2019;10:169. PubMed, PubMedCentral, CrossRef
- Hanahan D. Hallmarks of Cancer: New Dimensions. Cancer Discov. 2022;12(1):31-46. PubMed, CrossRef
- Hapach LA, Mosier JA, Wang W, Reinhart-King CA. Engineered models to parse apart the metastatic cascade. NPJ Precis Oncol. 2019;3:20. PubMed, PubMedCentral, CrossRef
- Wu B, Shi X, Jiang M, Liu H. Cross-talk between cancer stem cells and immune cells: potential therapeutic targets in the tumor immune microenvironment. Mol Cancer. 2023;22(1):38. PubMed, PubMedCentral, CrossRef
- Lei MML, Lee TKW. Cancer Stem Cells: Emerging Key Players in Immune Evasion of Cancers. Front Cell Dev Biol. 2021;9:692940. PubMed, PubMedCentral, CrossRef
- Verona F, Di Bella S, Schirano R, Manfredi C, Angeloro F, Bozzari G, Todaro M, Giannini G, Stassi G, Veschi V. Cancer stem cells and tumor-associated macrophages as mates in tumor progression: mechanisms of crosstalk and advanced bioinformatic tools to dissect their phenotypes and interaction. Front Immunol. 2025;16:1529847. PubMed, PubMedCentral, CrossRef
- Lee H, Kim B, Park J, Park S, Yoo G, Yum S, Kang W, Lee JM, Youn H, Youn B. Cancer stem cells: landscape, challenges and emerging therapeutic innovations. Signal Transduct Target Ther. 2025;10(1):248. PubMed, PubMedCentral, CrossRef
- Huang T, Song X, Xu D, Tiek D, Goenka A, Wu B, Sastry N, Hu B, Cheng SY. Stem cell programs in cancer initiation, progression, and therapy resistance. Theranostics. 2020;10(19):8721-8743. PubMed, PubMedCentral, CrossRef
- Kolling S, Ventre F, Geuna E, Milan M, Pisacane A, Boccaccio C, Sapino A, Montemurro F. “Metastatic Cancer of Unknown Primary” or “Primary Metastatic Cancer”? Front Oncol. 2020;9:1546. PubMed, PubMedCentral, CrossRef
- Gao P, Zou D, Zhao A, Yang P. Design and Optimization of the Circulatory Cell-Driven Drug Delivery Platform. Stem Cells Int. 2021;2021:8502021. PubMed, PubMedCentral, CrossRef
- Xiao G, Wang X, Xu Z, Liu Y, Jing J. Lung-specific metastasis: the coevolution of tumor cells and lung microenvironment. Mol Cancer. 2025;24(1):118. PubMed, PubMedCentral, CrossRef
- Zhao Q, Zong H, Zhu P, Su C, Tang W, Chen Z, Jin S. Crosstalk between colorectal CSCs and immune cells in tumorigenesis, and strategies for targeting colorectal CSCs. Exp Hematol Oncol. 2024;13(1):6. PubMed, PubMedCentral, CrossRef
- Bahmad HF, Cheaito K, Chalhoub RM, Hadadeh O, Monzer A, Ballout F, El-Hajj A, Mukherji D, Liu YN, Daoud G, Abou-Kheir W. Sphere-Formation Assay: Three-Dimensional in vitro Culturing of Prostate Cancer Stem/Progenitor Sphere-Forming Cells. Front Oncol. 2018;8:347. PubMed, PubMedCentral, CrossRef
- Kumar V, Vashishta M, Kong L, Wu X, Lu JJ, Guha C, Dwarakanath BS. The Role of Notch, Hedgehog, and Wnt Signaling Pathways in the Resistance of Tumors to Anticancer Therapies. Front Cell Dev Biol. 2021;9:650772. PubMed, PubMedCentral, CrossRef
- Groenewald W, Lund AH, Gay DM. The Role of WNT Pathway Mutations in Cancer Development and an Overview of Therapeutic Options. Cells. 2023;12(7):990. PubMed, PubMedCentral, CrossRef
- Li R, Banjanin B, Schneider RK, Costa IG. Detection of cell markers from single cell RNA-seq with sc2marker. BMC Bioinformatics. 2022;23(1):276. PubMed, PubMedCentral, CrossRef
- Song P, Gao Z, Bao Y, Chen L, Huang Y, Liu Y, Dong Q, Wei X. Wnt/β-catenin signaling pathway in carcinogenesis and cancer therapy. J Hematol Oncol. 2024;17(1):46. PubMed, PubMedCentral, CrossRef
- You M, Xie Z, Zhang N, Zhang Y, Xiao D, Liu S, Zhuang W, Li L, Tao Y. Signaling pathways in cancer metabolism: mechanisms and therapeutic targets. Signal Transduct Target Ther. 2023;8(1):196. PubMed, PubMedCentral, CrossRef
- Meisel CT, Porcheri C, Mitsiadis TA. Cancer Stem Cells, Quo Vadis? The Notch Signaling Pathway in Tumor Initiation and Progression. Cells. 2020;9(8):1879. PubMed, PubMedCentral, CrossRef
- Zhou B, Lin W, Long Y, Yang Y, Zhang H, Wu K, Chu Q. Notch signaling pathway: architecture, disease, and therapeutics. Signal Transduct Target Ther. 2022;7(1):95. PubMed, PubMedCentral, CrossRef
- Sachan N, Sharma V, Mutsuddi M, Mukherjee A. Notch signalling: multifaceted role in development and disease. FEBS J. 2024;291(14):3030-3059. PubMed, PubMedCentral, CrossRef
- Sari IN, Phi LTH, Jun N, Wijaya YT, Lee S, Kwon HY. Hedgehog Signaling in Cancer: A Prospective Therapeutic Target for Eradicating Cancer Stem Cells. Cells. 2018;7(11):208. PubMed, PubMedCentral, CrossRef
- Riobo-Del Galdo NA, Lara Montero Á, Wertheimer EV. Role of Hedgehog Signaling in Breast Cancer: Pathogenesis and Therapeutics. Cells. 2019;8(4):375. PubMed, PubMedCentral, CrossRef
- Zhang J, Fan J, Zeng X, Nie M, Luan J, Wang Y, Ju D, Yin K. Hedgehog signaling in gastrointestinal carcinogenesis and the gastrointestinal tumor microenvironment. Acta Pharm Sin B. 2021;11(3):609-620. PubMed, PubMedCentral, CrossRef
- Martelli AM, Paganelli F, Truocchio S, Palumbo C, Chiarini F, McCubrey JA. Understanding the Roles of the Hedgehog Signaling Pathway during T-Cell Lymphopoiesis and in T-Cell Acute Lymphoblastic Leukemia (T-ALL). Int J Mol Sci. 2023;24(3):2962. PubMed, PubMedCentral, CrossRef
- El-Tanani M, Rabbani SA, Satyam SM, Rangraze IR, Wali AF, El-Tanani Y, Aljabali AAA. Deciphering the Role of Cancer Stem Cells: Drivers of Tumor Evolution, Therapeutic Resistance, and Precision Medicine Strategies. Cancers (Basel). 2025;17(3):382. PubMed, PubMedCentral, CrossRef
- Lyu N, Pedersen B, Shklovskaya E, Rizos H, Molloy MP, Wang Y. SERS characterization of colorectal cancer cell surface markers upon anti-EGFR treatment. Exploration (Beijing). 2022;2(3):20210176. PubMed, PubMedCentral, CrossRef
- Teoh PL, Saini N. Biomarkers, isolation methods, and therapeutic implications of breast cancer stem cells. Cancer Pathog Ther. 2025;3(5):392-401. PubMed, PubMedCentral, CrossRef
- Sarabia-Sánchez MA, Tinajero-Rodríguez JM, Ortiz-Sánchez E, Alvarado-Ortiz E. Cancer Stem Cell markers: Symphonic masters of chemoresistance and immune evasion. Life Sci. 2024;355:123015. PubMed, CrossRef
- Varillas JI, Zhang J, Chen K, Barnes II, Liu C, George TJ, Fan ZH. Microfluidic Isolation of Circulating Tumor Cells and Cancer Stem-Like Cells from Patients with Pancreatic Ductal Adenocarcinoma. Theranostics. 2019;9(5):1417-1425. PubMed, PubMedCentral, CrossRef
- Radu P, Zurzu M, Paic V, Bratucu M, Garofil D, Tigora A, Georgescu V, Prunoiu V, Pasnicu C, Popa F, Surlin P, Surlin V, Strambu V. CD34-Structure, Functions and Relationship with Cancer Stem Cells. Medicina (Kaunas). 2023;59(5):938. PubMed, PubMedCentral, CrossRef
- McLean ME, MacLean MR, Cahill HF, Arun RP, Walker OL, Wasson MD, Fernando W, Venkatesh J, Marcato P. The Expanding Role of Cancer Stem Cell Marker ALDH1A3 in Cancer and Beyond. Cancers (Basel). 2023;15(2):492. PubMed, PubMedCentral, CrossRef
- Kim WT, Ryu CJ. Cancer stem cell surface markers on normal stem cells. BMB Rep. 2017;50(6):285-298. PubMed, PubMedCentral, CrossRef
- Qiu L, Li H, Fu S, Chen X, Lu L. Surface markers of liver cancer stem cells and innovative targeted-therapy strategies for HCC. Oncol Lett. 2018;15(2):2039-2048. PubMed, PubMedCentral, CrossRef
- Prasad S, Ramachandran S, Gupta N, Kaushik I, Srivastava SK. Cancer cells stemness: A doorstep to targeted therapy. Biochim Biophys Acta Mol Basis Dis. 2020;1866(4):165424. PubMed, PubMedCentral, CrossRef
- Gires O, Pan M, Schinke H, Canis M, Baeuerle PA. Expression and function of epithelial cell adhesion molecule EpCAM: where are we after 40 years? Cancer Metastasis Rev. 2020;39(3):969-987. PubMed, PubMedCentral, CrossRef
- Tacconelli E, Carrara E, Savoldi A, Harbarth S, Mendelson M, Monnet DL, Pulcini C, Kahlmeter G, Kluytmans J, Carmeli Y, Ouellette M, Outterson K, Patel J, Cavaleri M, Cox EM, Houchens CR, Grayson ML, Hansen P, Singh N, Theuretzbacher U, Magrini N; WHO Pathogens Priority List Working Group. Discovery, research, and development of new antibiotics: the WHO priority list of antibiotic-resistant bacteria and tuberculosis. Lancet Infect Dis. 2018;18(3):318-327. PubMed, CrossRef
- Herreros-Pomares A. Identification, Culture and Targeting of Cancer Stem Cells. Life (Basel). 2022;12(2):184. PubMed, PubMedCentral, CrossRef
- Croker H, Whitaker KL, Cooke L, Wardle J. Do social norms affect intended food choice? Prev Med. 2009;49(2-3):190-193. PubMed, CrossRef
- 5Ekström S, Andersson N, Lövquist A, Lauber A, Georgelis A, Kull I, Melén E, Bergström A. COVID-19 among young adults in Sweden: self-reported long-term symptoms and associated factors. Scand J Public Health. 2022;50(1):85-93. PubMed, PubMedCentral, CrossRef
- Mesbahi Y, Trahair TN, Lock RB, Connerty P. Exploring the Metabolic Landscape of AML: From Haematopoietic Stem Cells to Myeloblasts and Leukaemic Stem Cells. Front Oncol. 2022;12:807266. PubMed, PubMedCentral, CrossRef
- Hayashi Y, Nguyen VTT. A narrative review of imatinib-resistant gastrointestinal stromal tumors. Gastrointest Stromal Tumor. 2021;4:6. PubMed, PubMedCentral, CrossRef
- Fathy El-Metwaly N, Aref S, Ayed M, Abdel Hamid M, El-Sokkary AMA. CD34+/CD38- Stem Cell Burden Could Predict Chronic Myeloid Leukemia Patients’ Outcome. Asian Pac J Cancer Prev. 2021;22(10):3237-3243. PubMed, PubMedCentral, CrossRef
- Botezatu C, Costea DO, Nichilò M, Lazar AM, Andraș D, Radu MI, Mastalier B. The Five-Year Outcomes of Breast Cancer Surgical Management at the Colentina Surgical Clinic, Bucharest, Romania: A Descriptive Retrospective Analysis Between 2019 and 2023. Life (Basel). 2025;15(1):92. PubMed, PubMedCentral, CrossRef
- Kushwaha PP, Verma S, Kumar S, Gupta S. Role of prostate cancer stem-like cells in the development of antiandrogen resistance. Cancer Drug Resist. 2022;5(2):459-471. PubMed, PubMedCentral, CrossRef
- Parmiani G. Melanoma Cancer Stem Cells: Markers and Functions. Cancers (Basel). 2016;8(3):34. PubMed, PubMedCentral, CrossRef
- Wang J, Wu Y, Gao W, Li F, Bo Y, Zhu M, Fu R, Liu Q, Wen S, Wang B. Identification and characterization of CD133+CD44+ cancer stem cells from human laryngeal squamous cell carcinoma cell lines. J Cancer. 2017;8(3):497-506. PubMed, PubMedCentral, CrossRef
- Gupta P, Rizvi SZ, Lal N, Gupta V, Srivastav AN, Musa O. Expression of CD44 and CD133 stem cell markers in squamous cell carcinoma of esophagus. Indian J Pathol Microbiol. 2021;64(3):472-478. PubMed, CrossRef
- Natarajan K, Xie Y, Baer MR, Ross DD. Role of breast cancer resistance protein (BCRP/ABCG2) in cancer drug resistance. Biochem Pharmacol. 2012;83(8):1084-1103. PubMed, PubMedCentral, CrossRef
- Bhatia S, Wang P, Toh A, Thompson EW. New Insights Into the Role of Phenotypic Plasticity and EMT in Driving Cancer Progression. Front Mol Biosci. 2020;7:71. PubMed, PubMedCentral, CrossRef
- Garg P, Malhotra J, Kulkarni P, Horne D, Salgia R, Singhal SS. Emerging Therapeutic Strategies to Overcome Drug Resistance in Cancer Cells. Cancers (Basel). 2024;16(13):2478. PubMed, PubMedCentral, CrossRef
- Doustmihan A, Fathi M, Mazloomi M, Salemi A, Hamblin MR, Jahanban-Esfahlan R. Molecular targets, therapeutic agents and multitasking nanoparticles to deal with cancer stem cells: A narrative review. J Control Release. 2023;363:57-83. PubMed, CrossRef
- Aramini B, Masciale V, Grisendi G, Bertolini F, Maur M, Guaitoli G, Chrystel I, Morandi U, Stella F, Dominici M, Haider KH. Dissecting Tumor Growth: The Role of Cancer Stem Cells in Drug Resistance and Recurrence. Cancers (Basel). 2022;14(4):976. PubMed, PubMedCentral, CrossRef
- Liu YP, Zheng CC, Huang YN, He ML, Xu WW, Li B. Molecular mechanisms of chemo- and radiotherapy resistance and the potential implications for cancer treatment. MedComm. 2021;2(3):315-340. PubMed, PubMedCentral, CrossRef
- Manni W, Min W. Signaling pathways in the regulation of cancer stem cells and associated targeted therapy. MedComm. 2022;3(4):e176. PubMed, PubMedCentral, CrossRef
- Zeng Z, Fu M, Hu Y, Wei Y, Wei X, Luo M. Regulation and signaling pathways in cancer stem cells: implications for targeted therapy for cancer. Mol Cancer. 2023;22(1):172. PubMed, PubMedCentral, CrossRef
- Chen Y, Chen L, Yu J, Ghia EM, Choi MY, Zhang L, Zhang S, Sanchez-Lopez E, Widhopf GF 2nd, Messer K, Rassenti LZ, Jamieson C, Kipps TJ. Cirmtuzumab blocks Wnt5a/ROR1 stimulation of NF-κB to repress autocrine STAT3 activation in chronic lymphocytic leukemia. Blood. 2019;134(13):1084-1094. PubMed, PubMedCentral, CrossRef
- Choi MY, Widhopf GF 2nd, Ghia EM, Kidwell RL, Hasan MK, Yu J, Rassenti LZ, Chen L, Chen Y, Pittman E, Pu M, Messer K, Prussak CE, Castro JE, Jamieson C, Kipps TJ. Phase I Trial: Cirmtuzumab Inhibits ROR1 Signaling and Stemness Signatures in Patients with Chronic Lymphocytic Leukemia. Cell Stem Cell. 2018;22(6):951-959.e3. PubMed, PubMedCentral, CrossRef
- Cui X, Liu R, Duan L, Cao D, Zhang Q, Zhang A. CAR-T therapy: Prospects in targeting cancer stem cells. J Cell Mol Med. 2021;25(21):9891-9904. PubMed, PubMedCentral, CrossRef
- Li B, Kong X, Post H, Raaijmakers L, Peeper DS, Altelaar M. Proteomics and Phosphoproteomics Profiling of Drug-Addicted BRAFi-Resistant Melanoma Cells. J Proteome Res. 2021;20(9):4381-4392. PubMed, PubMedCentral, CrossRef
- Jung J, Seol HS, Chang S. The Generation and Application of Patient-Derived Xenograft Model for Cancer Research. Cancer Res Treat. 2018;50(1):1-10. PubMed, PubMedCentral, CrossRef
- Hadiloo K, Mostanadi P, Asadzadeh A, Taremi S, Esmaeilzadeh A. Targeting cancer stem cells with CAR-based immunotherapy: biology, evidence, and future directions. Cancer Cell Int. 2025;25(1):289. PubMed, PubMedCentral, CrossRef
- Masoumi J, Jafarzadeh A, Abdolalizadeh J, Khan H, Philippe J, Mirzaei H, Mirzaei HR. Cancer stem cell-targeted chimeric antigen receptor (CAR)-T cell therapy: Challenges and prospects. Acta Pharm Sin B. 2021;11(7):1721-1739. PubMed, PubMedCentral, CrossRef
- Scioli MG, Storti G, D’Amico F, Gentile P, Fabbri G, Cervelli V, Orlandi A. The Role of Breast Cancer Stem Cells as a Prognostic Marker and a Target to Improve the Efficacy of Breast Cancer Therapy. Cancers (Basel). 2019;11(7):1021. PubMed, PubMedCentral, CrossRef
This work is licensed under a Creative Commons Attribution 4.0 International License.







