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   93067篇
  免费   268篇
  国内免费   887篇
  94222篇
  2023年   2篇
  2022年   2篇
  2021年   3篇
  2019年   5篇
  2018年   11842篇
  2017年   10672篇
  2016年   7450篇
  2015年   585篇
  2014年   278篇
  2013年   290篇
  2012年   4197篇
  2011年   12810篇
  2010年   12000篇
  2009年   8218篇
  2008年   9777篇
  2007年   11355篇
  2006年   252篇
  2005年   498篇
  2004年   960篇
  2003年   1017篇
  2002年   772篇
  2001年   253篇
  2000年   160篇
  1999年   24篇
  1998年   7篇
  1997年   21篇
  1996年   9篇
  1994年   6篇
  1993年   29篇
  1992年   21篇
  1991年   37篇
  1990年   7篇
  1989年   8篇
  1988年   17篇
  1987年   13篇
  1985年   2篇
  1984年   7篇
  1983年   18篇
  1982年   2篇
  1980年   1篇
  1975年   5篇
  1972年   247篇
  1971年   274篇
  1970年   5篇
  1965年   13篇
  1962年   24篇
  1956年   5篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.

Introduction  

Rheumatoid arthritis (RA) is a chronic disease associated with inflammation and destruction of bone and cartilage. Although inhibition of TNFα is widely used to treat RA, a significant number of patients do not respond to TNFα blockade, and therefore there is a compelling need to continue to identify alternative therapeutic strategies for treating chronic inflammatory diseases such as RA. The anti-epidermal growth factor (anti-EGF) receptor antibody trastuzumab has revolutionised the treatment of patients with EGF receptor-positive breast cancer. Expression of EGF ligands and receptors (known as HER) has also been documented in RA. The highly unique compound RB200 is a bispecific ligand trap that is composed of full-length extracellular domains of HER1 and HER3 EGF receptors. Because of its pan-HER specificity, RB200 inhibits responses mediated by HER1, HER2 and HER3 in vitro and in vivo. The objective of this study was to assess the effect of RB200 combined with TNF blockade in a murine collagen-induced arthritis (CIA) model of RA.  相似文献   
282.

Background  

With the advance of microarray technology, several methods for gene classification and prognosis have been already designed. However, under various denominations, some of these methods have similar approaches. This study evaluates the influence of gene expression variance structure on the performance of methods that describe the relationship between gene expression levels and a given phenotype through projection of data onto discriminant axes.  相似文献   
283.
Elucidating the relationship between sequence and conformation is essential for the understanding of functions of proteins. While sharing 88 % sequence identity and differing by only seven residues, GA88 and GB88 have completely different structures and serve as ideal systems for investigating the relationship between sequence and function. Benefiting from the continuous advancement of the computational ability of modern computers, molecular dynamics (MD) simulation is now playing an increasingly important role in the study of proteins. However, the reliability of MD simulations is limited by the accuracy of the force fields and solvent model approximations. In this work, several AMBER force fields (AMBER03, AMBER99SB, AMBER12SB, AMBER14SB, AMBER96) and solvent models (TIP3P, IGB5, IGB7, IGB8) have been employed in the simulations of GA88 and GB88. The statistical results from 19 simulations show that GA88 and GB88 both adopt more compact structures than the native structures. GB88 is more stable than GA88 regardless of the force fields and solvent models utilized. Most of the simulations overestimated the salt bridge interaction. The combination of AMBER14SB force field and IGB8 solvent model shows the best overall performance in the simulations of both GA88 and GB88. AMBER03 and AMBER12SB also yield reasonable results but only in the TIP3P explicit solvent model.  相似文献   
284.
Stenolaemate bryozoans with their stable calcitic skeletons play a significant role in reef building. In the Middle Devonian Sabkhat Lafayrina reef complex (Western Sahara), bryozoans are abundant and diverse. Although they do not form part of the principal framework of reefs, bryozoans are involved significantly in reef growth, especially in the initial stage. In this way, bryozoans are important with respect to initiating reef growth. They contribute greatly to sediment stabilization, making it possible for principal reef builders to grow on hardened and stabilized substrates, and also play sediment-baffling and sediment-filling roles. The aim of this study is to document the diversity of bryozoans in a Middle Devonian reef complex and to estimate their potential for initiation and contribution to reef structures.  相似文献   
285.
Behavioural counselling by nurses can enhance quality of life in elderly with a personality disorder. Although nurses have a crucial role in day-to-day treatment, there is a lack of evidence-based approaches. Based on the cognitive therapy, the treatment protocol Cognitive Model for Behavioural Interventions (CoMBI) provides an alternative nursing approach for personality disorders.  相似文献   
286.
287.
288.
A cyanobacterial strain (Anabaena laxa RPAN8) exhibiting fungicidal activity and β-1,3 and 1,4 endoglucanase activities was selected for identifying the gene(s) involved. Functional analyses of the genomic library revealed that four clones (8, 64, 116, and 248) of RPAN8 exhibited fungicidal activity and induced structural deformities in the cell wall of the growing mycelia of Pythium aphanidermatum. Higher expression of fungicidal and β-1,4 endoglucanase activities, along with low expression of β-1,3 endoglucanase activity, was recorded in two E. coli clones (8 and 64). Clones 8 and 64 exhibited identical sequences while clones 116 and 248 were also similar. Bioinformatic analyses were undertaken only for the two non-identical clones 8 and 116 which showed open reading frames (ORFs) of 348 (end 1) and 656 amino acid residues (end 2), respectively. The amino acid sequence analyses revealed that the end 1 encoding endoglucanases belonged to peptidase M20 family while end 2 showed significant similarities with several known genes. The putative promoters and ribosomal binding sites were identified and amino acid exchanges were observed in both end 1 and 2. The presence of signal peptides of 24 and 20 amino acid residues respectively revealed the secretory nature of these proteins.  相似文献   
289.

Background  

Understanding the adaptive changes that alter the function of proteins during evolution is an important question for biology and medicine. The increasing number of completely sequenced genomes from closely related organisms, as well as individuals within species, facilitates systematic detection of recent selection events by means of comparative genomics.  相似文献   
290.
Dynamic landscape models have generally assumed random distributions of habitat although real landscapes show spatial organization at many scales. To explore the role of spatial structure in determining the frequency of dispersal-limited forest species, we used a cellular landscape model divided into two zones. Zones were distributed in a random, clustered, or regular spatial pattern. Within each zone habitat cells were randomly destroyed and regenerated, and habitat density and turnover rate were systematically varied. A hypothetical habitat-limited species dispersed between adjacent habitat cells. All trials showed a reduced species frequency relative to a static landscape. Reduction was greater at low habitat density (P = 0.30) than at high density (0.90) suggesting the importance of habitat connectivity in controlling species frequency. The greatest reduction occurred when habitat was concentrated in a small, regularly distributed zone at low habitat density reflecting the enforced isolation of individual habitat cells. Very little reduction was observed when habitat cells were packed into a small clustered zone, a situation promoting connectivity between cells. Moderate–severe frequency reduction occurred when habitat turnover was concentrated in a clustered zone at high habitat density, but little was observed when turnover was widely distributed in a regular or random pattern. These results can be interpreted in terms of a source-sink function in which spatial pattern controlled the degree of contact between landscape zones and determined opportunities for dispersal between habitat cells. We conclude that clustering of forest habitat has the potential to maintain herb species frequency in sparsely forested landscapes. Conversely, clustering of forest disturbance in heavily forested regions, or regular distribution of forest stands (as often occurs in agricultural regions) creates areas which are difficult to colonize, and should be avoided.  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