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   30996篇
  免费   84篇
  国内免费   152篇
系统科学   119篇
丛书文集   289篇
教育与普及   101篇
理论与方法论   88篇
现状及发展   13952篇
研究方法   1241篇
综合类   15101篇
自然研究   341篇
  2013年   219篇
  2012年   408篇
  2011年   710篇
  2008年   526篇
  2007年   597篇
  2006年   578篇
  2005年   588篇
  2004年   611篇
  2003年   576篇
  2002年   534篇
  2001年   968篇
  2000年   967篇
  1999年   617篇
  1994年   365篇
  1992年   566篇
  1991年   436篇
  1990年   530篇
  1989年   468篇
  1988年   459篇
  1987年   521篇
  1986年   530篇
  1985年   656篇
  1984年   453篇
  1983年   453篇
  1982年   408篇
  1981年   396篇
  1980年   418篇
  1979年   1002篇
  1978年   791篇
  1977年   725篇
  1976年   613篇
  1975年   686篇
  1974年   845篇
  1973年   747篇
  1972年   720篇
  1971年   904篇
  1970年   1094篇
  1969年   904篇
  1968年   925篇
  1967年   805篇
  1966年   729篇
  1965年   536篇
  1964年   201篇
  1959年   294篇
  1958年   531篇
  1957年   383篇
  1956年   336篇
  1955年   321篇
  1954年   322篇
  1948年   272篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
Prinz S  Amon A 《Nature》1999,402(6758):133, 135
  相似文献   
992.
993.
994.
This article describes the first results from the recent revision to The ONS classification of local and health authorities of Great Britain to allow for boundary changes. For each local and health authority of Great Britain as at April 1999 it presents 'most similar' authorities as measured by a selection of 37 socioeconomic and demographic variables from the 1991 Census. These results will assist local and health authorities in identifying other appropriate authorities for comparative studies. A second article, to be published in Population Trends 99, will present the results of the analysis to identify groups of authorities in a hierarchy of new Clusters, Groups and Families for the revised classification.  相似文献   
995.
Apolipoprotein E (apoE) ɛ4 allele is a genetic risk factor for late-onset familial and sporadic Alzheimer’s disease (AD). In the central nervous system, apoE is secreted mainly by astrocytes as a constituent of high-density lipoproteins. A recent study using apoE knockout mice provided strong evidence that apoE promotes cerebral deposition of amyloid β protein (Aβ). However, no clear explanation of the pathogenesis of apoE-induced AD has been provided. Here we discuss two possible mechanisms by which apoE might enhance Aβ deposition. One is the intracellular pathway in which apoE is internalized by neurons and induces lysosomal accumulation of Aβ and amyloidogenic APP (amyloid precursor protein) fragments, leading to neuronal death. The other is the extracellular pathway in which apoE-containing lipoproteins are trapped by Aβ1–42 deposits mobilizing soluble Aβ peptides and consequently enlarge amyloid plaques. These two mechanisms may operate at different stages of AD pathogenesis and suggest a chaperone-like function for the apoE molecule. Received 4 February 1999; received after revision 9 April 1999; accepted 23 April 1999  相似文献   
996.
Integrin antagonists   总被引:4,自引:0,他引:4  
Integrins are a family of cell surface glycoproteins that mediate numerous cell-cell and cell-matrix interactions and are involved in biological processes such as tissue morphogenesis, leukocyte recirculation and migration, wound healing, blood clotting and immune response. Aberrant cell adhesion has been implicated in the pathogenesis of several diseases, including a number of inflammatory disorders such as rheumatoid arthritis, inflammatory bowel disease and asthma, as well as cancer and coronary heart disease. As such integrins are seen as excellent targets for the development of therapeutic agents. This report begins with an examination of the structure of integrin molecules and their ligands and then goes on to review the current state of development of antiintegrin antagonists. Received 13 April 1999; received after revision 28 May 1999; accepted 28 May 1999  相似文献   
997.
998.
A better definition of the structural and thermodynamic determinants of the interaction of nucleic acids with proteins is shedding light on the origin of the genetic code, protein synthesis, and nucleic acid replication. This is also allowing to show a consistent biochemical framework for the appearance of these fundamental synthetic mechanisms. This article reviews recent significant developments in the field, and discusses an integrated model for a biochemically plausible evolution of these fundamental mechanisms of synthesis. This model is based on sequence-specific interactions between abiotically synthesized polynucleotides and polypeptides, and can account for a coordinate evolution of the genetic code, protein synthesis, and nucleic acid replication in living cells.  相似文献   
999.
Immune responses to DNA vaccines   总被引:16,自引:0,他引:16  
DNA vaccines, based on plasmid vectors expressing an antigen under the control of a strong promoter, have been shown to induce protective immune responses to a number of pathogens, including viruses, bacteria and parasites. They have also displayed efficacy in treatment or prevention of cancer, allergic diseases and autoimmunity. Immunologically, DNA vaccines induce a full spectrum of immune responses that include cytolytic T cells, T helper cells and antibodies. The immune response to DNA vaccines can be enhanced by genetic engineering of the antigen to facilitate its presentation to B and T cells. Furthermore, the immune response can be modulated by genetic adjuvants in the form of vectors expressing biologically active determinants or by more traditional adjuvants that facilitate uptake of DNA into cells. The ease of genetic manipulation of DNA vaccines invites their use not only as vaccines but also as research tools for immunologists and microbiologists. Received 26 October 1998; received after revision 3 December 1998; accepted 3 December 1998  相似文献   
1000.
Recent studies have suggested that aspirin and aspirin-like compounds have a variety of actions in addition to their well-studied ability to inhibit cyclooxygenases. These actions include inhibition of the uncoupling of oxidative phosphorylation, decreases in adenosine triphosphate stores, increases in extracellular adenosine, downregulation of the expression and activity of inducible nitric oxide synthetase, inhibition and/or stimulation of various mitogen-activated protein kinase activities and inhibition of nuclear factor binding κB site (NF-κB) activation. Moreover, aspirin-like compounds have recently been shown to have previously unappreciated clinical and biological effects, some apparently independent of cyclooxygenase. In this review we discuss the various mechanisms of action of aspirin-like compounds and their relevance to clinical disease and therapy. Received 1 February 1999; received after revision 1 April 1999; accepted 7 May 1999  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