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   28956篇
  免费   2559篇
  国内免费   2378篇
  2024年   57篇
  2023年   355篇
  2022年   885篇
  2021年   1526篇
  2020年   1141篇
  2019年   1345篇
  2018年   1284篇
  2017年   977篇
  2016年   1331篇
  2015年   1887篇
  2014年   2193篇
  2013年   2225篇
  2012年   2590篇
  2011年   2302篇
  2010年   1473篇
  2009年   1249篇
  2008年   1467篇
  2007年   1216篇
  2006年   1134篇
  2005年   901篇
  2004年   861篇
  2003年   815篇
  2002年   746篇
  2001年   606篇
  2000年   494篇
  1999年   487篇
  1998年   305篇
  1997年   281篇
  1996年   260篇
  1995年   239篇
  1994年   180篇
  1993年   159篇
  1992年   186篇
  1991年   156篇
  1990年   136篇
  1989年   98篇
  1988年   72篇
  1987年   68篇
  1986年   43篇
  1985年   34篇
  1984年   28篇
  1983年   33篇
  1982年   17篇
  1981年   11篇
  1980年   10篇
  1979年   8篇
  1978年   4篇
  1977年   5篇
  1974年   3篇
  1972年   3篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
961.
The changing profile of infection over time for Human Parechoviruses (HPeVs) is not well known and no detailed study has been reported to date in China. This investigation on HPeV infection in hospitalized children in Lanzhou, China revealed variations in epidemiological characteristics after a three-year interval. To assess the changes that had occurred, epidemiological and clinical characteristics of HPeVs were characterized and compared with previously reported data by our group. A comparable positivity rate (25.3%, 73/289) was revealed after the three-year interval with the majority of the infected children (95.9%, 70/73) being younger than two years of age. While a temporal change in the seasonal distribution was noted in the current study, HPeVs were more frequently detected during July to November compared to September to December in the previous study. Changes in HPeV genotypes patterns, a temporal change in the prevalence of HPeV1, a younger susceptible age to HPeV3 compared with HPeV1 and a tendency of older children to be infected with HPeV4 are in contrast to our previous report. HPeV2, a rarely reported genotype, was identified for the first time in China. In addition, an exclusive trinucleotide (GAT) insertion in the HPeV4 nucleotide sequence was identified. However, the profiles of co-infection with other enteric related viruses were similar to our previous findings. In summary, these data suggest temporal variation in the seasonal distribution of HPeV and changing patterns of HPeV genotypes over time in the study region.  相似文献   
962.
The incidence of tuberculosis (TB) and its risk factors in China remains unclear. This study examined TB incidence and relative risk factors in rural areas of China. Participants (n = 177,529) were recruited in Xiangtan County (in the central area of China) and in Danyang County (in the eastern area of China) in 2009 and a followed-up study was conducted for one year. The incidence density of pulmonary TB and smear-positive TB were 91.6 (95% CI: 78.7, 106.0) per 100,000 person-year and 36.7 (95% CI: 33.1, 52.4) per 100,000 person-year respectively in Xiangtan, and 47.3 (95% CI: 38.2, 57.5) per 100,000 person-year and 22.7 (95% CI: 16.5, 30.8) per 100,000 person-year in Danyang. The medical history of TB was associated with TB, with the relative risk (RR) of 7.00 (95% CI: 2.76, 17.18) in Xiangtan and that of 31.08 (95% CI: 13.22, 73.10) in Danyang. The association between TB and per capita living space over median was found in Xiangtan, with the RR of 1.86 (95% CI: 1.15, 3.01). No association was found between TB and the insurance status, the contact history with TB, the history of diabetes, smoking, or per capita annual income. The host genetic susceptibility, and social factors such as education and income could be considered in future studies.  相似文献   
963.
Berberine (BBR), an isoquinoline derivative alkaloid isolated from Chinese herbs, has a long history of uses for the treatment of multiple diseases, including cancers. However, the precise mechanisms of actions of BBR in human lung cancer cells remain unclear. In this study, we investigated the molecular mechanisms by which BBR inhibits cell growth in human non-small-cell lung cancer (NSCLC) cells. Treatment with BBR promoted cell morphology change, inhibited cell migration, proliferation and colony formation, and induced cell apoptosis. Further molecular mechanism study showed that BBR simultaneously targeted multiple cell signaling pathways to inhibit NSCLC cell growth. Treatment with BBR inhibited AP-2α and AP-2β expression and abrogated their binding on hTERT promoters, thereby inhibiting hTERT expression. Knockdown of AP-2α and AP-2β by siRNA considerably augmented the BBR-mediated inhibition of cell growth. BBR also suppressed the nuclear translocation of p50/p65 NF-κB proteins and their binding to COX-2 promoter, causing inhibition of COX-2. BBR also downregulated HIF-1α and VEGF expression and inhibited Akt and ERK phosphorylation. Knockdown of HIF-1α by siRNA considerably augmented the BBR-mediated inhibition of cell growth. Moreover, BBR treatment triggered cytochrome-c release from mitochondrial inter-membrane space into cytosol, promoted cleavage of caspase and PARP, and affected expression of BAX and Bcl-2, thereby activating apoptotic pathway. Taken together, these results demonstrated that BBR inhibited NSCLC cell growth by simultaneously targeting AP-2/hTERT, NF-κB/COX-2, HIF-1α/VEGF, PI3K/AKT, Raf/MEK/ERK and cytochrome-c/caspase signaling pathways. Our findings provide new insights into understanding the anticancer mechanisms of BBR in human lung cancer therapy.  相似文献   
964.

Background

Predicting protein subnuclear localization is a challenging problem. Some previous works based on non-sequence information including Gene Ontology annotations and kernel fusion have respective limitations. The aim of this work is twofold: one is to propose a novel individual feature extraction method; another is to develop an ensemble method to improve prediction performance using comprehensive information represented in the form of high dimensional feature vector obtained by 11 feature extraction methods.

Methodology/Principal Findings

A novel two-stage multiclass support vector machine is proposed to predict protein subnuclear localizations. It only considers those feature extraction methods based on amino acid classifications and physicochemical properties. In order to speed up our system, an automatic search method for the kernel parameter is used. The prediction performance of our method is evaluated on four datasets: Lei dataset, multi-localization dataset, SNL9 dataset and a new independent dataset. The overall accuracy of prediction for 6 localizations on Lei dataset is 75.2% and that for 9 localizations on SNL9 dataset is 72.1% in the leave-one-out cross validation, 71.7% for the multi-localization dataset and 69.8% for the new independent dataset, respectively. Comparisons with those existing methods show that our method performs better for both single-localization and multi-localization proteins and achieves more balanced sensitivities and specificities on large-size and small-size subcellular localizations. The overall accuracy improvements are 4.0% and 4.7% for single-localization proteins and 6.5% for multi-localization proteins. The reliability and stability of our classification model are further confirmed by permutation analysis.

Conclusions

It can be concluded that our method is effective and valuable for predicting protein subnuclear localizations. A web server has been designed to implement the proposed method. It is freely available at http://bioinformatics.awowshop.com/snlpred_page.php.  相似文献   
965.

Background

Autism is a neurodevelopmental disorder with a high estimated heritability. ATP2B2, located on human chromosome 3p25.3, encodes the plasma membrane calcium-transporting ATPase 2 which extrudes Ca2+ from cytosol into extracellular space. Recent studies reported association between ATP2B2 and autism in samples from Autism Genetic Resource Exchange (AGRE) and Italy. In this study, we investigated whether ATP2B2 polymorphisms were associated with autism in Chinese Han population.

Methods

We performed a family based association study between five SNPs (rs35678 in exon, rs241509, rs3774180, rs3774179, and rs2278556 in introns) in ATP2B2 and autism in 427 autism trios of Han Chinese descent. All SNPs were genotyped using the Sequenom genotyping platform. The family-based association test (FBAT) program was used to perform association test for SNPs and haplotype analyses.

Results

This study demonstrated a preferential transmission of T allele of rs3774179 to affected offsprings under an additive model (T>C, Z = 2.482, p = 0.013). While C allele of rs3774179 showed an undertransmission from parents to affected children under an additive and a dominant model, respectively (Z = −2.482, p = 0.013; Z = −2.591, p = 0.0096). Haplotype analyses revealed that three haplotypes were significantly associated with autism. The haplotype C-C (rs3774180–rs3774179) showed a significant undertransmission from parents to affected offsprings both in specific and global haplotype FBAT (Z = −2.037, p = 0.042; Global p = 0.03). As for the haplotype constructed by rs3774179 and rs2278556, C-A might be a protective haplotype (Z = −2.206, p = 0.027; Global p = 0.04), while T-A demonstrated an excess transmission from parents to affected offsprings (Z = 2.143, p = 0.032). These results were still significant after using the permutation method to obtain empirical p values.

Conclusions

Our research suggested that ATP2B2 might play a role in the etiology of autism in Chinese Han population.  相似文献   
966.

Background

Cancer cells typically exhibit large-scale aberrant methylation of gene promoters. Some of the genes with promoter methylation alterations play “driver” roles in tumorigenesis, whereas others are only “passengers”.

Results

Based on the assumption that promoter methylation alteration of a driver gene may lead to expression alternation of a set of genes associated with cancer pathways, we developed a computational framework for integrating promoter methylation and gene expression data to identify driver methylation aberrations of cancer. Applying this approach to breast cancer data, we identified many novel cancer driver genes and found that some of the identified driver genes were subtype-specific for basal-like, luminal-A and HER2+ subtypes of breast cancer.

Conclusion

The proposed framework proved effective in identifying cancer driver genes from genome-wide gene methylation and expression data of cancer. These results may provide new molecular targets for potential targeted and selective epigenetic therapy.  相似文献   
967.
Recent genetic-based studies have implicated a number of immune-related genes in the pathogenesis of inflammatory bowel disease (IBD). Our recent genetic studies showed that RAC2 is associated with human IBD; however, its role in disease pathogenesis is unclear. Given Rac2’s importance in various fundamental immune cell processes, we investigated whether a defect in Rac2 may impair host immune responses in the intestine and promote disease in the context of an infection-based (Citrobacter rodentium) model of colitis. In response to infection, Rac2−/− mice showed i) worsened clinical symptoms (days 13–18), ii) increased crypt hyperplasia at days 11 and 22 (a time when crypt hyperplasia was largely resolved in wild-type mice; WT), and iii) marked mononuclear cell infiltration characterized by higher numbers of T (CD3+) cells (day 22), compared to WT-infected mice. Moreover, splenocytes harvested from infected Rac2−/− mice and stimulated in vitro with C. rodentium lysate produced considerably higher levels of interferon-γ and interleukin-17A. The augmented responses observed in Rac2−/− mice did not appear to stem from Rac2’s role in NADPH oxidase-driven reactive oxygen species production as no differences in crypt hyperplasia, nor inflammation, were observed in infected NOX2−/− mice compared to WT. Collectively, our findings demonstrate that Rac2−/− mice develop more severe disease when subjected to a C. rodentium-induced model of infectious colitis, and suggest that impaired Rac2 function may promote the development of IBD in humans.  相似文献   
968.

Objective

The aims of current study were to assess the level of tobacco knowledge, anti-tobacco messages and major information channels in Zhejiang.

Methods

Face-to-face interviews were conducted with 2112 adults in Zhejiang. Data on demographic information, smoking status, tobacco knowledge, anti-tobacco messages and major information channels was collected.

Results

The findings revealed that only 31.87% of the population were aware that smoking could cause all three diseases (stoke, heart disease, and lung cancer), 86.09% were aware that smoking causes lung cancer, 46.43% and 42.40% were aware that smoking causes stroke and heart attack, respectively. Residence and education level had significant effects on awareness, while the effects of smoking status, gender, age, and household monthly income were not significant. There were five major information channels as follows: television (67.52%), newspapers or magazines (40.79%), billboards (30.02%), public walls (24.72), and radio (23.79%). Respondents got the following anti-tobacco messages from mass media: “No smoking in public” (66.34%), “No smoking in front of other people” (35.18%) and “Not offering cigarettes to one another” (22.82%).

Conclusions

The tobacco knowledge among residents in Zhejiang province is relatively poor. Improved information channels and content of anti-tobacco messages are necessary to increase the public’s tobacco knowledge, particularly among rural residents and people with less education.  相似文献   
969.

Background

Rapidly growing evidence suggests that microRNAs (miRNAs) are involved in a wide range of cancer malignant behaviours including radioresistance. Therefore, the present study was designed to investigate miRNA expression patterns associated with radioresistance in NPC.

Methods

The differential expression profiles of miRNAs and mRNAs associated with NPC radioresistance were constructed. The predicted target mRNAs of miRNAs and their enriched signaling pathways were analyzed via biological informatical algorithms. Finally, partial miRNAs and pathways-correlated target mRNAs were validated in two NPC radioreisitant cell models.

Results

50 known and 9 novel miRNAs with significant difference were identified, and their target mRNAs were narrowed down to 53 nasopharyngeal-/NPC-specific mRNAs. Subsequent KEGG analyses demonstrated that the 53 mRNAs were enriched in 37 signaling pathways. Further qRT-PCR assays confirmed 3 down-regulated miRNAs (miR-324-3p, miR-93-3p and miR-4501), 3 up-regulated miRNAs (miR-371a-5p, miR-34c-5p and miR-1323) and 2 novel miRNAs. Additionally, corresponding alterations of pathways-correlated target mRNAs were observed including 5 up-regulated mRNAs (ICAM1, WNT2B, MYC, HLA-F and TGF-β1) and 3 down-regulated mRNAs (CDH1, PTENP1 and HSP90AA1).

Conclusions

Our study provides an overview of miRNA expression profile and the interactions between miRNA and their target mRNAs, which will deepen our understanding of the important roles of miRNAs in NPC radioresistance.  相似文献   
970.
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