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   616篇
  免费   103篇
  国内免费   9篇
  2021年   7篇
  2020年   5篇
  2019年   8篇
  2018年   6篇
  2017年   8篇
  2016年   11篇
  2015年   27篇
  2014年   22篇
  2013年   32篇
  2012年   29篇
  2011年   37篇
  2010年   18篇
  2009年   26篇
  2008年   20篇
  2007年   30篇
  2006年   21篇
  2005年   24篇
  2004年   22篇
  2003年   26篇
  2002年   16篇
  2001年   21篇
  2000年   12篇
  1999年   21篇
  1998年   16篇
  1997年   13篇
  1996年   20篇
  1995年   9篇
  1994年   13篇
  1993年   11篇
  1992年   13篇
  1991年   10篇
  1990年   7篇
  1989年   5篇
  1988年   10篇
  1987年   5篇
  1986年   10篇
  1985年   10篇
  1984年   6篇
  1983年   6篇
  1982年   5篇
  1978年   6篇
  1977年   4篇
  1975年   6篇
  1974年   5篇
  1972年   7篇
  1969年   4篇
  1956年   7篇
  1955年   5篇
  1948年   4篇
  1930年   4篇
排序方式: 共有728条查询结果,搜索用时 15 毫秒
41.
42.
Effective bioterrorism planning, prevention, and response require information sharing between various entities, ranging from public health authorities and health-care workers to national security and law enforcement officials. While the source of much information exchanged may be nonidentifiable, many entities legitimately need access to personally identifiable health information (or "protected health information" [PHI]) in planning for and responding to a bioterrorism event. The HIPAA Privacy Rule allows for essential exchanges of health data during a public health emergency while protecting against unnecessary disclosures of PHI. In the event of a bioterrorist attack, the Privacy Rule allows covered entities to disclose PHI without individual authorization in the following instances: (1) for treatment by health-care providers, (2) to avert a serious threat to health or safety, (3) to public health authorities for public health purposes, (4) to protect national security, (5) to law enforcement under certain conditions, and (6) for judicial or administrative proceedings. Despite these favorable disclosure provisions, some privacy challenges remain. The flow of PHI may be slowed by misunderstandings of the Privacy Rule's accounting requirement. In addition, in a bioterrorism scenario, nontraditional entities may find themselves acting as health-care providers, triggering Privacy Rule provisions. Finally, the potential for de facto disclosures of individuals' disease or exposure status increases where conspicuous treatment methods, isolation, or quarantine are implemented without additional measures to protect privacy. Understanding the Privacy Rule's impact on bioterrorism planning and response ensures that various entities can conduct their activities with needed information while still protecting individual privacy.  相似文献   
43.

Background  

Structural genomics (SG) projects aim to determine thousands of protein structures by the development of high-throughput techniques for all steps of the experimental structure determination pipeline. Crucial to the success of such endeavours is the careful tracking and archiving of experimental and external data on protein targets.  相似文献   
44.
The chemokine receptor CCR9 is expressed on most small intestinal lamina propria and intraepithelial lymphocytes and on a small subset of peripheral blood lymphocytes. CCR9-expressing lymphocytes may play an important role in small bowel immunity and inflammation. We studied the phenotype and functional characteristics of CCR9(+) lymphocytes in blood from normal donors. A subset of CCR9(+) T cells have a phenotype of activated cells and constitutively express the costimulatory molecules CD40L and OX-40. In contrast to CCR9(-), CCR9(+)CD4(+) peripheral blood T cells proliferate to anti-CD3 or anti-CD2 stimulation and produce high levels of IFN-gamma and IL-10. IL-10-producing cells were exclusively detected within the CCR9(+) subset of CD4(+) T cells by intracellular staining and were distinct from IL-2- and IFN-gamma-producing cells. Moreover, memory CCR9(+)CD4(+) lymphocytes respond to CD2 stimulation with proliferation and IFN-gamma/IL-10 production, whereas memory CCR9(-)CD4(+) cells were unresponsive. In addition, memory CCR9(+)CD4(+) T cells support Ig production by cocultured CD19(+) B cells in the absence of prior T cell activation or addition of exogenous cytokines. Our data show that the memory subset of circulating CCR9(+)CD4(+) T cells has characteristics of mucosal T lymphocytes and contains cells with either Th1 or T-regulatory 1 cytokine profiles. Studies on the cytokine profile and Ag specificity of this cell subset could provide important insight into small intestinal immune-mediated diseases and oral tolerance in humans.  相似文献   
45.
CD8(+) CTL play important roles against malignancy in both active and passive immunotherapy. Nonetheless, the success of antitumor CTL responses may be improved by additional therapeutic modalities. Radiotherapy, which has a long-standing use in treating neoplastic disease, has been found to induce unique biologic alterations in cancer cells affecting Fas gene expression, which, consequently, may influence the overall lytic efficiency of CTL. Here, in a mouse adenocarcinoma cell model, we examined whether exposure of these tumor cells to sublethal doses of irradiation 1) enhances Fas expression, leading to more efficient CTL killing via Fas-dependent mechanisms in vitro; and 2) improves antitumor activity in vivo by adoptive transfer of these Ag-specific CTL. Treatment of carcinoembryonic Ag-expressing MC38 adenocarcinoma cells with irradiation (20 Gy) in vitro enhanced Fas expression at molecular, phenotypic, and functional levels. Furthermore, irradiation sensitized these targets to Ag-specific CTL killing via the Fas/Fas ligand pathway. We examined the effect of localized irradiation of s.c. growing tumors on the efficiency of CTL adoptive immunotherapy. Irradiation caused up-regulation of Fas by these tumor cells in situ, based on immunohistochemistry. Moreover, localized irradiation of the tumor significantly potentiated tumor rejection by these carcinoembryonic Ag-specific CTL. Overall, these results showed for the first time that 1) regulation of the Fas pathway in tumor cells by irradiation plays an important role in their sensitization to Ag-specific CTL; and 2) a combination regimen of tumor-targeted irradiation and CTL promotes more effective antitumor responses in vivo, which may have implications for the combination of immunotherapy and radiation therapy.  相似文献   
46.
The shape and nucleation of primary conidia are important characters in the classification of the Entomophthoraceae (Zygomycetes). The five species in the genus Eryniopsis vary in the shapes of primary conidia, although within most genera in the order Entomophthorales species have the same shapes of primary conidia. Using PCR-RFLP, we investigated two species in Eryniopsis, Ery. caroliniana with oblong-ovoid primary conidia and Ery. ptychopterae with pear-shaped primary conidia, with five species of Entomophaga, all having pear-shaped conidia. Molecular results merged with morphological data indicate that Ery. ptychopterae belongs in the genus Entomophaga while Ery. caroliniana clearly differs from Entomophaga. Ery. ptychopterae and Ery. transitans are transferred to the genus Entomophaga. Our results support the idea that morphology of primary conidia is of major importance in defining entomophthoralean genera. These results also show that such studies can be conducted with species that have not been isolated, if fungal-filled cadavers can be obtained.  相似文献   
47.
Heterologous desensitization of chemokine receptors by opioids has been considered to contribute to their immunosuppressive effects. Previous studies show that Met-enkephalin, an endogenous opioid, down-regulates chemotaxis of selected chemokine receptors via phosphorylation. In the present study, we further investigated the molecular mechanism of such cross-regulation. Our data showed that preincubation with Met-enkephalin inhibited both MIP-1 alpha-mediated chemotaxis and Ca(2+) flux of monocytes in a dose-dependent manner. The inhibitory effects were maximal using nanomolar concentrations of activating chemokines, a concentration found in physiological conditions. A decrease both in chemokine receptor affinity and in coupling efficiency between receptors and G protein were observed, which directly contributed to the desensitization effects. However, comparing with chemokines such as MIP-1 alpha and MCP-1, opioids did not elicit a calcium flux, failed to induce MIP-1 alpha receptors internalization, and mediated a less potent heterologous desensitization. We hypothesized that these differences might originate from the involvement of different protein kinase C (PKC) isotypes. In our studies, opioid-mediated down-regulation of MIP-1 alpha receptors could be blocked by the general PKC inhibitor calphostin C, but not by the calcium-dependent classic PKC inhibitor Go6976. Western blotting analysis and immunofluorescent staining further showed that only calcium-independent PKCs were activated upon opioid stimulation. Thus, opioids achieve desensitization of chemokine receptors via a unique pathway, involving only calcium-independent PKC isotypes.  相似文献   
48.
During early seed development, nuclear divisions in the endosperm are not followed by cell division, leading to the development of a syncytium. The simple organization of the Arabidopsis endosperm provides a model in which to study the regulation of the cell cycle in relation to development. To monitor nuclear divisions, we constructed a HISTONE 2B::YELLOW FLUORESCENT PROTEIN gene fusion (H2B::YFP). To validate its use as a vital marker for chromatin in plants, H2B::YFP was expressed constitutively in Arabidopsis. This enabled the observation of mitoses in living root meristems. H2B::YFP was expressed specifically in Arabidopsis syncytial endosperm by using GAL4 transactivation. Monitoring mitotic activity in living syncytial endosperm showed that the syncytium was organized into three domains in which nuclei divide simultaneously with a specific time course. Each mitotic domain has a distinct spatiotemporal pattern of mitotic CYCLIN B1;1 accumulation. The polar spatial organization of the three mitotic domains suggests interactions between developmental mechanisms and the regulation of the cell cycle.  相似文献   
49.
The North American native species Drosophila pseudoobscura was first identified in New Zealand in the last few decades. Here, we have studied the genetic consequences of its spread across the Pacific Ocean. Using 10 microsatellites that are highly variable in North American populations, we found that the New Zealand population has substantially fewer alleles, a much lower average heterozygosity, and significantly different allele frequencies at these loci. We have discussed the relative sensitivity of these parameters for detecting the founding event. X-linked loci were more strongly differentiated between continents than autosomal loci, as reflected by larger changes in allele frequencies and greater reductions in numbers of alleles and average heterozygosity. The severity of the genetic diversity loss supports a scenario of a few D. pseudoobscura females being introduced to New Zealand from North America.  相似文献   
50.
A method using a strong anion-exchange liquid-chromatography column, Mono-Q, has been developed for high-resolution analysis and purification of oligonucleotide dithioates, which were synthesized by an automated, solid-phase, phosphorothioamidite chemistry. High-resolution separation of oligonucleotide phosphorodithioates from monothiophosphate impurities was obtained. High-resolution separation was also demonstrated at pH 8. The separation of oligonucleotide dithioates was found to be linearly dependent on the number of sulfurs for the same sequence length. Thiocyanate, SCN-, as eluting anion, can be used to purify oligonucleotides containing a high percentage of phosphorodithioate linkages in lower salt concentrations and provides better separation than chloride as eluting anion.  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