首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2414篇
  免费   192篇
  国内免费   29篇
  2635篇
  2023年   23篇
  2022年   65篇
  2021年   73篇
  2020年   45篇
  2019年   53篇
  2018年   70篇
  2017年   64篇
  2016年   79篇
  2015年   95篇
  2014年   118篇
  2013年   131篇
  2012年   164篇
  2011年   160篇
  2010年   92篇
  2009年   63篇
  2008年   104篇
  2007年   83篇
  2006年   93篇
  2005年   83篇
  2004年   63篇
  2003年   54篇
  2002年   55篇
  2001年   59篇
  2000年   58篇
  1999年   47篇
  1998年   21篇
  1997年   18篇
  1996年   15篇
  1994年   12篇
  1993年   23篇
  1992年   39篇
  1991年   44篇
  1990年   41篇
  1989年   39篇
  1988年   37篇
  1987年   39篇
  1986年   39篇
  1985年   30篇
  1984年   17篇
  1983年   16篇
  1982年   19篇
  1981年   22篇
  1980年   10篇
  1979年   31篇
  1978年   20篇
  1977年   14篇
  1976年   15篇
  1975年   10篇
  1973年   15篇
  1971年   11篇
排序方式: 共有2635条查询结果,搜索用时 15 毫秒
991.
When TT virus (TTV) DNA was quantitated in whole blood and plasma aliquots from 27 viremic individuals by real-time detection PCR that can detect essentially all TTV genotypes, the TTV load was 6.9 +/- 3.5 (mean +/- standard deviation)-fold higher in the whole blood than in the plasma samples [P < 0.002 (paired t test)]. To clarify the reason for this difference, peripheral blood cells of various types including red blood cells, granulocytes (CD15+), B cells (CD19+), T cells (CD3+), monocytes (CD14+), and NK cells (CD3-/CD56+) were separated at a purity of 95.4-99.5% from each of three infected individuals with relatively high TTV viremia, and their TTV viral loads were determined. Red blood cells were uniformly negative, but the other cell types were positive for TTV DNA at various titers. In all three patients, the highest TTV load was found in granulocytes (4.2 x 10(4)-3.1 x 10(5) copies/10(6) cells), followed by monocytes (1.4-2.2 x 10(4) copies/10(6) cells) and NK cells (5.4-6.5 x 10(3) copies/10(6) cells); B and T cells were positive, with a low viral load (6.7 x 10(1)-2.7 x 10(3) copies/10(6) cells). These results indicate that TTV is distributed in various peripheral blood cell types at distinct levels, with the highest viral load in granulocytes, and that a significant proportion of the TTV DNA in peripheral blood is not identified by the standard plasma/serum DNA detection methods.  相似文献   
992.
A requirement for generating transgenic pigeonpea [Cajanuscajan (L.) Millsp] plants is the development of a highly efficientin vitro regeneration procedure. This goal was achieved byusing germinated seedlings grown on B5 medium supplemented with 10 mgl–1 6-benzylaminopurine, which induced differentiatingcallus formation in the cotyledonary node region. The calli were transferred onB5 medium with 0.2 mg l–1 6-benzylaminopurine toobtain shoot induction. Elongated shoots were then further cultured on a B5hormone-free medium for rooting. Using this regeneration system transgenicpigeonpea plants were obtained both by particle bombardment andAgrobacterium tumefaciens-mediated gene transfer. Thepresence of the transgenes in the pigeonpea genome was confirmed by GUS assays,PCR and Southern hybridisation. The transgenic rooted plants were successfullytransferred to soil in the greenhouse. GUS and PCR assays of T1 progeniesconfirmed that the transgenes were stably transmitted to the next generation.This is the first report of successful use ofAgrobacteriumas well as particle bombardment for production of transgenic pigeonpea plants.  相似文献   
993.
994.
Highly pathogenic H5N1 influenza A viruses have spread across Asia, Europe, and Africa. More than 500 cases of H5N1 virus infection in humans, with a high lethality rate, have been reported. To understand the molecular basis for the high virulence of H5N1 viruses in mammals, we tested the virulence in ferrets of several H5N1 viruses isolated from humans and found A/Vietnam/UT3062/04 (UT3062) to be the most virulent and A/Vietnam/UT3028/03 (UT3028) to be avirulent in this animal model. We then generated a series of reassortant viruses between the two viruses and assessed their virulence in ferrets. All of the viruses that possessed both the UT3062 hemagglutinin (HA) and nonstructural protein (NS) genes were highly virulent. By contrast, all those possessing the UT3028 HA or NS genes were attenuated in ferrets. These results demonstrate that the HA and NS genes are responsible for the difference in virulence in ferrets between the two viruses. Amino acid differences were identified at position 134 of HA, at positions 200 and 205 of NS1, and at positions 47 and 51 of NS2. We found that the residue at position 134 of HA alters the receptor-binding property of the virus, as measured by viral elution from erythrocytes. Further, both of the residues at positions 200 and 205 of NS1 contributed to enhanced type I interferon (IFN) antagonistic activity. These findings further our understanding of the determinants of pathogenicity of H5N1 viruses in mammals.  相似文献   
995.
The heterogeneity of breast cancer requires the discovery of more incisive molecular tools that better define disease progression and prognosis. Proteomic analysis of homogeneous tumor cell populations derived by laser microdissection from formalin-fixed, paraffin-embedded (FFPE) tissues has proven to be a robust strategy for conducting retrospective cancer biomarker investigations. We describe an MS-based analysis of laser microdissected cancerous epithelial cells derived from twenty-five breast cancer patients at defined clinical disease stages with the goal of identifying protein abundance characteristics indicative of disease progression and recurrence. Comparative analysis of stage 0 and stage III patients revealed 113 proteins that significantly differentiated these groups and included known factors associated with disease pathogenesis, such as CDH1 and CTNNB1, as well as those previously implicated in breast cancer, such as TSP-1. Similar analyses of patients presenting with stage II disease that did or did not exhibit recurrence two years postdiagnosis revealed 42 proteins that significantly differentiated these subgroups and included IRS-1 and PARK7. These data provide evidence supporting the utility of FFPE tissues for functional proteomic analyses and protein biomarker discovery and yielded protein candidates indicative of disease stage and recurrence in breast cancer that warrant further investigation for diagnostic utility and biological relevance.  相似文献   
996.
The interaction between human endothelial cells and leukocytes during immunologic and inflammatory responses is in part mediated through the release of soluble mediators. We report that cultured human umbilical vein endothelial cells secrete IL-6 when stimulated with LPS. This effect was inhibited by polymyxin-B. The monokines IL-1 and TNF-alpha were also potent inducers of IL-6, whereas lymphotoxin was only effective at much higher concentrations. Endothelial cell supernatant IL-6 was active as hybridoma-plasmacytoma growth factor and as B-cell stimulating factor. Endothelial IL-6 activity was neutralized by a specific anti-IL-6 antibody and by immunoprecipitation it was shown to be identical in size to human fibroblast-derived IL-6. As IL-6 is possibly an important regulator of host defense responses, production of this cytokine by endothelial cells may contribute to the pathogenesis of various inflammatory and immunologic diseases.  相似文献   
997.
Animals gather information about their environment from a variety of sources to enable adaptive decision-making behaviour. Eavesdropping on heterospecific alarm calls enhances predator avoidance, reduces time spent vigilant and allows for more time on daily activities such as foraging. If the information is relevant and reliable, individuals that respond to heterospecific signals may benefit from a wider range of information at a low marginal cost. The Cape ground squirrel (Xerus inauris) and crowned lapwing (Vanellus chilensis) are ground-dwelling species that are taxonomically distant but share similar predators, habitat and anti-predatory behaviours. We used playback experiments of the alarm calls produced by conspecifics and lapwings to investigate the vigilance responses of adult female Cape ground squirrels. Squirrels responded with greater vigilance to both squirrel and lapwing alarm calls, and no changes of vigilance levels were observed in response to a control sound. However, contrary to our predictions, changes in vigilance and time to relax did not differ between conspecific versus heterospecific playbacks. The results from our study suggest that squirrels perceive lapwing alarm calls as relevant and reliable information and that responding to it could increase their survival.  相似文献   
998.
Among multiple subspecies of the protein kinase C (PKC) family, type I PKC from the hypothalamus, having the structure related to the gamma-sequence, responds to low concentrations of arachidonic acid to exhibit marked enzymatic activity. This mode of activation does not require elevated Ca2+ levels, nor does it depend on diacylglycerol and phospholipid. Type I PKC is expressed only in limited regions of central nervous tissues, such as the hypothalamus. This PKC subspecies is not detected in the pituitary gland. The results suggest that the activation of type I PKC may not always be directly associated with inositol phospholipid hydrolysis, and that this subspecies may play a role in the modulation of specialized functions of the hypothalamus.  相似文献   
999.
Escherichia coli cells that express the full six carotenoid biosynthesis genes (crtE, crtB, crtI, crtY, crtZ, and crtX) of the bacterium Pantoea ananatis have been shown to biosynthesize zeaxanthin 3,3′-β-d-diglucoside. We found that this recombinant E. coli also produced a novel carotenoid glycoside that contained a rare carbohydrate moiety, quinovose (chinovose; 6-deoxy-d-glucose), which was identified as 3-β-glucosyl-3′-β-quinovosyl zeaxanthin by chromatographic and spectroscopic analyses. The chirality of the aglycone of these zeaxanthin glycosides had been shown to be 3R,3′R, in which the hydroxyl groups were formed with the CrtZ enzyme. It was here demonstrated that zeaxanthin synthesized from β-carotene with CrtR or CYP175A1, the other hydroxylase with similar catalytic function to CrtZ, possessed the same stereochemistry. It was also suggested that the singlet oxygen-quenching activity of zeaxanthin 3,3′-β-d-diglucoside, which has a chemical structure close to the new carotenoid glycoside, was superior to that of zeaxanthin.  相似文献   
1000.

Background

Several studies have investigated whether the polymorphisms in the prostaglandin endoperoxide synthase 1 (PTGS1) and PTGS2 genes and nonsteroidal anti-inflammatory drug (NSAID) use are associated with cancer risk; however, those studies have produced mixed results. Therefore, we performed a meta-analysis to evaluate the association between the PTGS1 and PTGS2 polymorphisms and the effect of NSAID use on the risk of developing cancer.

Methods

We conducted a comprehensive search in PubMed through March 2012. The odds ratios (ORs) with the corresponding 95% confidence intervals (CIs) were calculated using the fixed-effect model or the random-effect model.

Results

The database search generated 13 studies that met the inclusion criteria. For PTGS1 rs3842787, NSAID users homozygous for the major allele (CC) had a significantly decreased cancer risk compared with non-NSAID users (OR = 0.73, 95% CI = 0.59–0.89). For PTGS2 rs5275 and rs20417, there were no significant differences between the gene polymorphism and NSAID use on cancer risk among the 8 and 7 studies, respectively. However, in the stratified analysis by the type of cancer or ethnicity population, NSAID users homozygous for the major allele (TT) in rs5275 demonstrated significantly decreased cancer risk compared with non-NSAID users in cancer type not involving colorectal adenoma (OR = 0.70, 95% CI = 0.59–0.83) and among the USA population (OR = 0.67, 95% CI = 0.56–0.82). NSAID users homozygous for the major allele (GG) in rs20417 displayed a significantly decreased cancer risk than non-NSAID users among the US population (OR = 0.72, 95% CI = 0.58–0.88). For the PTGS2 rs689466 and rs2745557 SNPs, there were no significant differences.

Conclusion

This meta-analysis suggests that the associations between PTGS polymorphisms and NSAID use on cancer risk may differ with regard to the type of cancer and nationality.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号