Journal of Oncology

Journal of Oncology / 2009 / Article

Clinical Study | Open Access

Volume 2009 |Article ID 139590 | 5 pages |

Prognostic Significance and Clinicopathological Associations of COX-2 SNP in Patients with Nonsmall Cell Lung Cancer

Academic Editor: Frederick E. Domann
Received21 May 2009
Accepted04 Sep 2009
Published22 Nov 2009


Background. To further improve the screening, diagnosis, and therapy of patients with nonsmall cell lung cancer (NSCLC) additional diagnostic tools are urgently needed. Gene expression of Cyclooxygenase-2 (COX-2) has been linked to prognosis in patients with NSCLC. The role of the COX-2 926G>C Single Nucleotide Polymorphism (SNP) in patients with NSCLC remains unclear. The aim of this study was to investigate the potential of the COX-2 926G>C SNP as a molecular marker in this disease. Methods. COX-2 926G>C SNP was analyzed in surgically resected tumor tissue of 85 patients with NSCLC using a PCR-based RFLP technique. Results. The COX-2 926G>C SNP genotypes were detected with the following frequencies: GG (73%), GC (23%), CC (4%). There were no associations between COX-2 SNP genotype and histology, grading or gender detectable. COX-2 SNP was significantly associated with tumor stage ( ) and lymph node status ( , Chi-square test). With a median followup of 85.9 months, the median survival was 59.7 months. There were no associations seen between the COX-2 SNP genotype and patients prognosis. Conclusions. The COX-2 926G>C SNP is detectable at a high frequency in patients with NSCLC. The COX-2 926G>C SNP genotype is not a prognostic molecular marker in this disease. However, patients with the GC or CC genotype seem more susceptible to lymph node metastases and higher tumor stage than patients with the GG genotype. The results suggest COX-2 926G>C SNP as a molecular marker for lymph node involvement in this disease.

1. Introduction

The smoking of tobacco is the most prevalent cause of lung cancer, which is the leading cause of cancer mortality in the world towards the end of the 20th century [1, 2]. Each year approximately 200 000 new cases of lung cancer are diagnosed in the United States, and there were over 160 000 lung cancer related deaths in 2008 [3]. The only treatment to cure patients with nonsmall cell lung cancer (NSCLC) is radical surgery, but the 5-year survival rate still remains poor. However, the concept of individualized treatment based on genetic differences among patients promises to provide improved treatment outcomes. Thus, the molecular mechanisms involved in carcinogenesis and pharmacogenetics have to be studied so that tailored treatments can be discovered and developed.

Inflammation has been recognized as a contributing factor in pathogenesis of many cancers [4]. Epidemiologic studies have shown that prolonged use of nonsteroidal anti-inflammatory drugs (NSAIDs) reduces the risk of a variety of cancers including lung cancer [58]. Cyclooxygenases (COXs, also named prostaglandin endoperoxide synthases or PTGSs) are the key enzymes in the conversion of arachidonic acid to prostaglandin (PG) and other eicosanoids [9, 10]. Two isoforms have been identified; COX-1 is consistently expressed in nearly all cells whereas COX-2 is normally undetectable but induced under circumstances such as inflammation and cancer [11]. Overexpression of COX-2 has been reported in several cancers, such as colorectal [12, 13], pancreatic [14], breast [15], esophageal [16], gastric [17], lung [18, 19], and several other cancers [2023].

Single nucleotide polymorphisms (SNPs) are common in the human genetic pool, and there is growing evidence suggesting that genetic polymorphisms play a role in the variability of drug response and toxicity in patients. A predictive value concerning the response to chemotherapy treatment has been reported for certain genetic SNPs in several tumors, for example, gastric cancer [24], breast cancer, colorectal cancer [2527], and lung cancer [28, 29]. Earlier studies have already shown that COX-2 SNPs have an impact in promoter activity and therefore influence the variability of response [30]. Papafili et al. reported a transcription alteration of the COX-2 gene caused by the COX-2 926G>C SNP in the promoter region and an increase of the levels of C-reactive protein.

The COX-2 926G>C SNP has been investigated earlier regarding a possible increased risk of developing NSCLC, but no association could be found for this SNP [31]. No study of COX-2 926G>C polymorphism and potential prognostic significance in NSCLC cancer patients has been reported so far. Hence the rationale for conducting this study was to investigate a possible prognostic role of the COX 926G>C SNP in NSCLC.

2. Material and Methods

2.1. Patients

85 tumor specimens from NSCLC patients, available from a previous prospective clinical trail of 103 consecutive patients [32], were included in this study. 65 patients were male (76%) and 20 female (24%) with the median age of 62.4 years. Seventy-six (89%) of these patients were smokers.

According to the International Union Against Cancer (UICC) TNM classification [33], 42 patients (49%) were tumor stage I, 18 patients (21%) stage II, and 25 patients (30%) stage IIIA. 39 (46%) patients had squamous cell carcinoma, 31 (36%) had adenocarcinoma, and 15 (18%) had large cell carcinoma. All 85 patients underwent thoracic surgery, and the tumors were R0 resected. Patients with histopathologic stage 3a tumors received postoperative radiotherapy. Informed consent was obtained from each patient.

The median followup was 85.9 months (range 63–105), and no patient was lost to followup. Tissue for gene expression analysis was obtained during surgery immediately after lung resection and before starting mediastinal lymphadenectomy. The tissues were immediately frozen in liquid nitrogen and stored at −80°C. Six-micrometer frozen sections were taken from blocks of tumor tissue. Starting with the first section, every fifth section was routinely stained with hematoxylin and eosin and evaluated histopathologically. Sections were pooled for analysis from areas estimated to have at least 75% malignant cells.

The primary tumors were graded histopathologically as well differentiated (G1, one patient), moderately differentiated (G2, eighteen patients), and poorly differentiated (G3, sixty-six patients).

2.2. DNA Extraction and Genotyping of the COX-2 926G>C SNP

DNA was extracted from representative tumor sections using the QIAamp DNA Mini Kit (Qiagen, Hilden, Germany). The COX-2 926G>C polymorphism was analysed in tumor tissue of 85 patients with NSCLC using a PCR-based RFLP technique. Forward and reverse primers used were as follows: -CAT TTA GCG TCC CTG CAA AT- and -TAC CTT CAC CCC CTC CTT GT- . Briefly, an approximately 2 ng DNA was added to a reaction volume of 15  L, containing 7.5  L TaqMan Universal PCR Master Mix, No AmpErase UNG, and 0.75  L custom-designed probe. Amplifications and determination of genotypes were performed using an Applied Biosystems 7500 Real Time PCR System as follows: 95°C ( ), 45 cycles of 93°C ( ), and 60°C ( ). PCR fragments were digested using 3 units of the restriction enzyme AciI and separated on a 3% agarose gel. A technician blinded for the clinical data performed PCR/RFLP analyses. A random sample of 20% of each polymorphism was repeated and showed 100% concordance.

2.3. Statistical Analysis

Statistical analyses were carried out using SPSS for Windows (version 17.0). A χ2 test was used to assess the association between categorical clinicopathologic data and COX-2 926G>C SNP genotype. Hazard ratios were used to calculate the relative risks of death. These calculations were based on the Pike estimate, with the use of the observed and expected number of events as calculated in the log-rank test statistic. The log-rank test [34] and Kaplan-Meier plots [35] were used to evaluate the association of genotypes and overall survival. Multivariate analysis was performed with the Cox proportional hazards regression model. Statistical significance was interpreted as . All -values reported were based on two-sided tests.

3. Results

In the studied cohort, the COX-2 926G>C SNP genotypes were detected with the following disposition: the wild type (WT) GG in (73%), the heterozygote SNP GC in (23%), and the homozygote SNP CC in (4%).

No association between COX-2 926G>C SNP genotype and histology was seen, even though the CC genotype ( ) was only found in squamous cell carcinoma patients.

Also, neither grading nor gender had any detectable association with the COX-2 926G>C SNP. All three patients with the genotype CC were male, but due to the small number of the CC genotype there was no statistical significance. Seventy six (89%) of the patients were smokers. There was no association between smokers and different COX 926 polymorphisms. However, in nonsmokers, the COX 926 polymorphism is more frequent than in smokers with borderline significance ( Pearson χ2; Fisher’s Exact Test) (Figure 1).

The COX-2 926G>C SNP was significantly associated with a higher tumor stage ( , Pearson χ2 test) (Figure 2). All three CC genotypes were stage IIIa, 5 CG and 13 GG genotypes were stage II, and 7 CG and 35 GG genotypes were found in stage I.

Also, associations were discovered in the COX-2 926G>C polymorphism and the lymph node metastasis ( , χ2 test) (Figure 3). The three patients with the CC genotype had all lymph node metastasis, two were pN1 and one was pN2. Fifteen out of 20 patients (75%) with the CG genotype had lymph node metastasis. Six of these patients were pN1 and seven pN2. Only 35% of the patients (22 of 62) with the GG genotype were suffering from lymph node metastasis.

With a median followup of 85.9 months, the median survival was 59.7 months (range 38–105 months). Neither the log-rank test (Mantel-Cox) ( ) nor the Kaplan-Meier plots (Figure 4) showed any prognostic significance for the COX-2 926G>C SNP.

4. Discussion

The COX-2 pathway is important in cancer development because it is involved in the regulation of various critical cellular processes such as tumor progression, metastases, angiogenesis, and chemotherapy resistance [3639]. Elevated COX-2 expression has been associated with poor prognoses in lung [4042] and other cancers, such breast [43], head and neck [44], colon [45], and cervix carcinomas [46]. However, little is known about COX-2 single nucleotide polymorphisms in NSCLC. In this study, we found that the COX-2 926G>C SNP is detectable at a high frequency in patients with NSCLC. We used PCR-based RFLP protocols to analyze the COX-2 926 genotype and found that 73% of the patients had the wild type genotype (GG), 23% were heterozygote (GC) and 4% homozygote (CC) for the COX-2 926 polymorphism.

Several previous studies have examined the associations of COX-2 polymorphisms and tumor diseases. In breast cancer, the COX-2 169-GG genotype was associated with increased risk [47], but the COX-2 926G>C SNP was not [48], while some tenuous evidence was found for an interaction between the C allele of the COX-2 8473 SNP with NSAIDs to reduce risk for hormone receptor positive breast cancer. In the case of NSCLC, it was suggested that the COX-2 8473SNP is associated with an increased risk of developing lung cancer [31] but COX-2 926G>C SNP was not, as already mentioned in the introduction.

The association between smoking and the COX-2 926G>C SNP did not reach statistical significance in this study but the trend suggests the need for further investigation with larger numbers.

Although we did not find the COX-2 926G>C SNP to be a prognostic marker for NSCLC, NSCLC patients with the GC or CC genotype were apparently more susceptible to lymph node metastases and higher tumor stage than patients with the GG genotype, suggesting that the COX-2 926G>C SNP is a molecular marker for lymph node involvement.


  1. S. S. Hecht, “Cigarette smoking and lung cancer: chemical mechanisms and approaches to prevention,” Lancet Oncology, vol. 3, no. 8, pp. 461–469, 2002. View at: Publisher Site | Google Scholar
  2. D. M. Parkin, F. Bray, J. Ferlay, and P. Pisani, “Global cancer statistics, 2002,” CA: A Cancer Journal for Clinicians, vol. 55, no. 2, pp. 74–108, 2005. View at: Google Scholar
  3. American Cancer Society, “Cancer Facts & Figures 2008,” American Cancer Society, Atlanta, Ga, USA, 2008. View at: Google Scholar
  4. L. M. Coussens and Z. Werb, “Inflammation and cancer,” Nature, vol. 420, no. 6917, pp. 860–867, 2002. View at: Publisher Site | Google Scholar
  5. C. Siemes, L. E. Visser, J. W. W. Coebergh, A. Hofman, A. G. Uitterlinden, and B. H. C. Stricker, “Protective effect of NSAIDs on cancer and influence of COX-2 C-765G genotype,” Current Cancer Drug Targets, vol. 8, no. 8, pp. 753–764, 2008. View at: Publisher Site | Google Scholar
  6. E. J. Jacobs, M. J. Thun, E. B. Bain, C. Rodriguez, S. J. Henley, and E. E. Calle, “A large cohort study of long-term daily use of adult-strength aspirin and cancer incidence,” Journal of the National Cancer Institute, vol. 99, no. 8, pp. 608–615, 2007. View at: Publisher Site | Google Scholar
  7. M. J. Thun, M. M. Namboodiri, and C. W. Heath Jr., “Aspirin use and reduced risk of fatal colon cancer,” The New England Journal of Medicine, vol. 325, no. 23, pp. 1593–1596, 1991. View at: Google Scholar
  8. R. E. Harris, J. Beebe-Donk, H. Doss, and D. Burr Doss, “Aspirin, ibuprofen, and other non-steroidal anti-inflammatory drugs in cancer prevention: a critical review of non-selective COX-2 blockade (review),” Oncology Reports, vol. 13, no. 4, pp. 559–583, 2005. View at: Google Scholar
  9. J. R. Vane, Y. S. Bakhle, and R. M. Botting, “Cyclooxygenases 1 and 2,” Annual Review of Pharmacology and Toxicology, vol. 38, pp. 97–120, 1998. View at: Google Scholar
  10. J. R. Vane and R. M. Botting, “Mechanism of action of aspirin-like drugs,” Seminars in Arthritis and Rheumatism, vol. 26, no. 6, supplement 2, pp. 2–10, 1997. View at: Google Scholar
  11. A. Koki, N. K. Khan, B. M. Woerner et al., “Cyclooxygenase-2 in human pathological disease,” Advances in Experimental Medicine and Biology, vol. 507, pp. 177–184, 2002. View at: Google Scholar
  12. C. E. Eberhart, R. J. Coffey, A. Radhika, F. M. Giardiello, S. Ferrenbach, and R. N. Dubois, “Up-regulation of cyclooxygenase 2 gene expression in human colorectal adenomas and adenocarcinomas,” Gastroenterology, vol. 107, no. 4, pp. 1183–1188, 1994. View at: Google Scholar
  13. H. Sano, Y. Kawahito, R. L. Wilder et al., “Expression of cyclooxygenase-1 and -2 in human colorectal cancer,” Cancer Research, vol. 55, no. 17, pp. 3785–3789, 1995. View at: Google Scholar
  14. O. N. Tucker, A. J. Dannenberg, E. K. Yang et al., “Cyclooxygenase-2 expression is up-regulated in human pancreatic cancer,” Cancer Research, vol. 59, no. 5, pp. 987–990, 1999. View at: Google Scholar
  15. D. Hwang, D. Scollard, J. Byrne, and E. Levine, “Expression of cyclooxygenase-1 and cyclooxygenase-2 in human breast cancer,” Journal of the National Cancer Institute, vol. 90, no. 6, pp. 455–460, 1998. View at: Google Scholar
  16. K. C. Zimmermann, M. Sarbia, A.-A. Weber, F. Borchard, H. E. Gabbert, and K. Schror, “Cyclooxygenase-2 expression in human esophageal carcinoma,” Cancer Research, vol. 59, no. 1, pp. 198–204, 1999. View at: Google Scholar
  17. A. Ristimaki, N. Honkanen, H. Jankala, P. Sipponen, and M. Harkonen, “Expression of cyclooxygenase-2 in human gastric carcinoma,” Cancer Research, vol. 57, no. 7, pp. 1276–1280, 1997. View at: Google Scholar
  18. R. A. Soslow, A. J. Dannenberg, D. Rush et al., “COX-2 is expressed in human pulmonary, colonic, and mammary tumors,” Cancer, vol. 89, no. 12, pp. 2637–2645, 2000. View at: Google Scholar
  19. H. Wolff, K. Saukkonen, S. Anttila, A. Karjalainen, H. Vainio, and A. Ristimaki, “Expression of cyclooxygenase-2 in human lung carcinoma,” Cancer Research, vol. 58, no. 22, pp. 4997–5001, 1998. View at: Google Scholar
  20. P. Uotila, E. Valve, P. Martikainen, M. Nevalainen, M. Nurmi, and P. Harkonen, “Increased expression of cyclooxygenase-2 and nitric oxide synthase-2 in human prostate cancer,” Urological Research, vol. 29, no. 1, pp. 25–28, 2001. View at: Publisher Site | Google Scholar
  21. S. Kulkarni, J. S. Rader, F. Zhang et al., “Cyclooxygenase-2 is overexpressed in human cervical cancer,” Clinical Cancer Research, vol. 7, no. 2, pp. 429–434, 2001. View at: Google Scholar
  22. T. Shirahama, “Cyclooxygenase-2 expression is up-regulated in transitional cell carcinoma and its preneoplastic lesions in the human urinary bladder,” Clinical Cancer Research, vol. 6, no. 6, pp. 2424–2430, 2000. View at: Google Scholar
  23. B. J. Tong, J. Tan, L. Tajeda et al., “Heightened expression of cyclooxygenase-2 and peroxisome proliferator-activated receptor-delta human endometrial adenocarcinoma,” Neoplasia, vol. 2, no. 6, pp. 483–490, 2000. View at: Google Scholar
  24. E. Goekkurt, S. Hoehn, C. Wolschke et al., “Polymorphisms of glutathione S-transferases (GST) and thymidylate synthase (TS)—novel predictors for response and survival in gastric cancer patients,” British Journal of Cancer, vol. 94, no. 2, pp. 281–286, 2006. View at: Publisher Site | Google Scholar
  25. E. Marcuello, A. Altes, E. del Rio, A. Cesar, A. Menoyo, and M. Baiget, “Single nucleotide polymorphism in the 5 tandem repeat sequences of Thymidylate synthase gene predicts for response to fluorouracil-based chemotherapy in advanced colorectal cancer patients,” International Journal of Cancer, vol. 112, no. 5, pp. 733–737, 2004. View at: Publisher Site | Google Scholar
  26. V. Cohen, V. Panet-Raymond, N. Sabbaghian, I. Morin, G. Batist, and R. Rozen, “Methylenetetrahydrofolate reductase polymorphism in advanced colorectal cancer: a novel genomic predictor of clinical response to fluoropyrimidine-based chemotherapy,” Clinical Cancer Research, vol. 9, no. 5, pp. 1611–1615, 2003. View at: Google Scholar
  27. J. Stoehlmacher, D. J. Park, W. Zhang et al., “A multivariate analysis of genomic polymorphisms: prediction of clinical outcome to 5-FU/oxaliplatin combination chemotherapy in refractory colorectal cancer,” British Journal of Cancer, vol. 91, no. 2, pp. 344–354, 2004. View at: Google Scholar
  28. W. Zhou, S. Gurubhagavatula, G. Liu et al., “Excision repair cross-complementation group 1 polymorphism predicts overall survival in advanced non-small cell lung cancer patients treated with platinum-based chemotherapy,” Clinical Cancer Research, vol. 10, no. 15, pp. 4939–4943, 2004. View at: Publisher Site | Google Scholar
  29. J.-H. Pan, J.-X. Han, J.-M. Wu, L.-J. Sheng, H.-N. Huang, and Q.-Z. Yu, “MDR1 single nucleotide polymorphisms predict response to vinorelbine-based chemotherapy in patients with non-small cell lung cancer,” Respiration, vol. 75, no. 4, pp. 380–385, 2008. View at: Publisher Site | Google Scholar
  30. A. Papafili, M. R. Hill, D. J. Brull et al., “Common promoter variant in cyclooxygenase-2 represses gene expression: evidence of role in acute-phase inflammatory response,” Arteriosclerosis, Thrombosis, and Vascular Biology, vol. 22, no. 10, pp. 1631–1636, 2002. View at: Publisher Site | Google Scholar
  31. D. Campa, S. Zienolddiny, V. Maggini, V. Skaug, A. Haugen, and F. Canzian, “Association of a common polymorphism in the cyclooxygenase 2 gene with risk of non-small cell lung cancer,” Carcinogenesis, vol. 25, no. 2, pp. 229–235, 2004. View at: Publisher Site | Google Scholar
  32. P. M. Schneider, H. W. Praeuer, O. Stoeltzing et al., “Multiple molecular marker testing (p53, C-Ki-ras, c-erbB-2) improves estimation of prognosis in potentially curative resected non-small cell lung cancer,” British Journal of Cancer, vol. 83, no. 4, pp. 473–479, 2000. View at: Google Scholar
  33. C. F. Mountain, “Revisions in the international system for staging lung cancer,” Chest, vol. 111, no. 6, pp. 1710–1717, 1997. View at: Google Scholar
  34. R. Miller and D. Siegmund, “Maximally selected chi square statistics,” Biometrics, vol. 38, no. 4, pp. 1011–1016, 1982. View at: Google Scholar
  35. E. Kaplan and P. Meier, “Nonparametric estimation from incomplete observations,” The Journal of the American Medical Association, vol. 53, pp. 457–481, 1958. View at: Google Scholar
  36. A. J. Dannenberg and K. Subbaramaiah, “Targeting cyclooxygenase-2 in human neoplasia: rationale and promise,” Cancer Cell, vol. 4, no. 6, pp. 431–436, 2003. View at: Publisher Site | Google Scholar
  37. D. S. Dandekar and B. L. Lokeshwar, “Inhibition of cyclooxygenase (COX)-2 expression by tet-inducible COX-2 antisense cDNA in hormone-refractory prostate cancer significantly slows tumor growth and improves efficacy of chemotherapeutic drugs,” Clinical Cancer Research, vol. 10, no. 23, pp. 8037–8047, 2004. View at: Publisher Site | Google Scholar
  38. K. Kishi, S. Petersen, C. Petersen et al., “Preferential enhancement of tumor radioresponse by a cyclooxygenase-2 inhibitor,” Cancer Research, vol. 60, no. 5, pp. 1326–1331, 2000. View at: Google Scholar
  39. M. Oshima, J. E. Dinchuk, S. L. Kargman et al., “Suppression of intestinal polyposis in Apc(delta716) knockout mice by inhibition of cyclooxygenase 2 (COX-2),” Cell, vol. 87, no. 5, pp. 803–809, 1996. View at: Publisher Site | Google Scholar
  40. F. R. Khuri, H. Wu, J. J. Lee et al., “Cyclooxygenase-2 overexpression is a marker of poor prognosis in stage I non-small cell lung cancer,” Clinical Cancer Research, vol. 7, no. 4, pp. 861–867, 2001. View at: Google Scholar
  41. A. Yuan, C.-J. Yu, C.-T. Shun et al., “Total cyclooxygenase-2 mRNA levels correlate with vascular endothelial growth factor mRNA levels, tumor angiogenesis and prognosis in non-small cell lung cancer patients,” International Journal of Cancer, vol. 115, no. 4, pp. 545–555, 2005. View at: Publisher Site | Google Scholar
  42. J. Brabender, J. Park, R. Metzger et al., “Prognostic significance of cyclooxygenase 2 mRNA expression in non-small cell lung cancer,” Annals of Surgery, vol. 235, no. 3, pp. 440–443, 2002. View at: Publisher Site | Google Scholar
  43. A. Ristimaki, A. Sivula, J. Lundin et al., “Prognostic significance of elevated cyclooxygenase-2 expression in breast cancer,” Cancer Research, vol. 62, no. 3, pp. 632–635, 2002. View at: Google Scholar
  44. O. Gallo, E. Masini, B. Bianchi, L. Bruschini, M. Paglierani, and A. Franchi, “Prognostic significance of cyclooxygenase-2 pathway and angiogenesis in head and neck squamous cell carcinoma,” Human Pathology, vol. 33, no. 7, pp. 708–714, 2002. View at: Publisher Site | Google Scholar
  45. H. Konno, M. Baba, T. Shoji, M. Ohta, S. Suzuki, and S. Nakamura, “Cyclooxygenase-2 expression correlates with uPAR levels and is responsible for poor prognosis of colorectal cancer,” Clinical and Experimental Metastasis, vol. 19, no. 6, pp. 527–534, 2002. View at: Publisher Site | Google Scholar
  46. G. Ferrandina, L. Lauriola, G. F. Zannoni et al., “Expression of cyclooxygenase-2 (COX-2) in tumour and stroma compartments in cervical cancer: clinical implications,” British Journal of Cancer, vol. 87, no. 10, pp. 1145–1152, 2002. View at: Publisher Site | Google Scholar
  47. F. Li, G. S. Ren, H. Y. Li, X. Y. Wang, L. Chen, and J. Li, “A novel single nucleotide polymorphism of the cyclooxygenase-2 gene associated with breast cancer,” Clinical Oncology, vol. 21, no. 4, pp. 302–305, 2009. View at: Publisher Site | Google Scholar
  48. J. Shen, M. D. Gammon, M. B. Terry, S. L. Teitelbaum, A. I. Neugut, and R. M. Santella, “Genetic polymorphisms in the cyclooxygenase-2 gene, use of nonsteroidal anti-inflammatory drugs, and breast cancer risk,” Breast Cancer Research, vol. 8, no. 6, article R71, 2006. View at: Publisher Site | Google Scholar

Copyright © 2009 Peter P. Grimminger et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

More related articles

1026 Views | 2353 Downloads | 5 Citations
 PDF  Download Citation  Citation
 Download other formatsMore
 Order printed copiesOrder

Related articles

We are committed to sharing findings related to COVID-19 as quickly and safely as possible. Any author submitting a COVID-19 paper should notify us at to ensure their research is fast-tracked and made available on a preprint server as soon as possible. We will be providing unlimited waivers of publication charges for accepted articles related to COVID-19. Sign up here as a reviewer to help fast-track new submissions.