全文获取类型
收费全文 | 15411篇 |
免费 | 1451篇 |
国内免费 | 4篇 |
专业分类
16866篇 |
出版年
2023年 | 90篇 |
2022年 | 227篇 |
2021年 | 434篇 |
2020年 | 265篇 |
2019年 | 297篇 |
2018年 | 346篇 |
2017年 | 302篇 |
2016年 | 488篇 |
2015年 | 846篇 |
2014年 | 906篇 |
2013年 | 993篇 |
2012年 | 1333篇 |
2011年 | 1410篇 |
2010年 | 879篇 |
2009年 | 738篇 |
2008年 | 956篇 |
2007年 | 944篇 |
2006年 | 891篇 |
2005年 | 718篇 |
2004年 | 785篇 |
2003年 | 683篇 |
2002年 | 674篇 |
2001年 | 136篇 |
2000年 | 77篇 |
1999年 | 110篇 |
1998年 | 150篇 |
1997年 | 86篇 |
1996年 | 75篇 |
1995年 | 77篇 |
1994年 | 78篇 |
1993年 | 96篇 |
1992年 | 60篇 |
1991年 | 54篇 |
1990年 | 47篇 |
1989年 | 34篇 |
1988年 | 37篇 |
1987年 | 38篇 |
1986年 | 28篇 |
1985年 | 35篇 |
1984年 | 43篇 |
1983年 | 27篇 |
1982年 | 44篇 |
1981年 | 36篇 |
1980年 | 33篇 |
1979年 | 22篇 |
1978年 | 33篇 |
1977年 | 17篇 |
1976年 | 21篇 |
1975年 | 15篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Background
Aberrant CD40 ligand (CD154) expression occurs on both T cells and B cells in human lupus patients, which is suggested to enhance B cell CD40 signaling and play a role in disease pathogenesis. Transgenic mice expressing CD154 by their B cells (CD154TG) have an expanded spleen B cell pool and produce autoantibodies (autoAbs). CD22 deficient (CD22−/−) mice also produce autoAbs, and importantly, their B cells are hyper-proliferative following CD40 stimulation ex vivo. Combining these 2 genetic alterations in CD154TGCD22−/− mice was thereby predicted to intensify CD40 signaling and autoimmune disease due to autoreactive B cell expansion and/or activation.Methodology/Principal Findings
CD154TGCD22−/− mice were assessed for their humoral immune responses and for changes in their endogenous lymphocyte subsets. Remarkably, CD154TGCD22−/− mice were not autoimmune, but instead generated minimal IgG responses against both self and foreign antigens. This paucity in IgG isotype switching occurred despite an expanded spleen B cell pool, higher serum IgM levels, and augmented ex vivo B cell proliferation. Impaired IgG responses in CD154TGCD22−/− mice were explained by a 16-fold expansion of functional, mature IL-10-competent regulatory spleen B cells (B10 cells: 26.7×106±6 in CD154TGCD22−/− mice; 1.7×106±0.4 in wild type mice, p<0.01), and an 11-fold expansion of B10 cells combined with their ex vivo-matured progenitors (B10+B10pro cells: 66×106±3 in CD154TGCD22−/− mice; 6.1×106±2 in wild type mice, p<0.01) that represented 39% of all spleen B cells.Conclusions/Significance
These results demonstrate for the first time that the IL-10-producing B10 B cell subset has the capacity to suppress IgG humoral immune responses against both foreign and self antigens. Thereby, therapeutic agents that drive regulatory B10 cell expansion in vivo may inhibit pathogenic IgG autoAb production in humans. 相似文献992.
993.
Inteins are autocatalytic protein domains that post-translationally excise from protein precursors and ligate their flanking regions with a peptide bond, in a process called protein splicing. Intein-containing DNA polymerases of cyanobacteria and nanoarchaea are naturally split into two separate genes at their intein domain. Such naturally occurring split inteins rapidly self-associate and reconstitute protein-splicing activity in trans. Here, we analyze the in vitro protein-splicing activity of three naturally split inteins from diverse cyanobacteria: Oscillatoria limnetica, Thermosynechococcus vulcanus, and Nostoc sp. PCC7120. N- and C-terminal halves of these split inteins were mixed in nine combinations, resulting in three endogenous (wild-type) and six exogenous combinations. Protein splicing was detected in all split-intein combinations, despite a 30-50% sequence variation between the homologous proteins. Splicing activity proceeded under a variety of conditions, including the presence of denaturants and reductants and high temperature, ionic strength, and viscosity. Still, in a high concentration of salt (2 M) or urea (6 M), specific combinations spliced significantly better than others. Additionally, copper ions were found to inhibit trans splicing in a reversible double-lock reaction. Our comparative analysis of naturally split inteins in endogenous and exogenous combinations demonstrates the modularity of trans protein-splicing elements and their robust activity. It suggests tight interactions between split-intein halves and conditions for modifying the specificity of intein parts. These results promote the biotechnological use of split inteins for controlled assembly of protein fragments either in vivo or in vitro and under moderate or extreme conditions. 相似文献
994.
Hanneke Borgdorff Stuart D. Armstrong Hanne L. P. Tytgat Dong Xia Gilles F. Ndayisaba Jonathan M. Wastling Janneke H. H. M. van de Wijgert 《PloS one》2016,11(3)
Background
A Lactobacillus-dominated cervicovaginal microbiota (VMB) protects women from adverse reproductive health outcomes, but the role of L. iners in the VMB is poorly understood. Our aim was to explore the association between the cervicovaginal L. iners and L. crispatus proteomes and VMB composition.Methods
The vaginal proteomes of 50 Rwandan women at high HIV risk, grouped into four VMB groups (based on 16S rDNA microarray results), were investigated by mass spectrometry using cervicovaginal lavage (CVL) samples. Only samples with positive 16S results for L. iners and/or L. crispatus within each group were included in subsequent comparative protein analyses: Lactobacillus crispatus-dominated VMB cluster (with 16S-proven L. iners (ni) = 0, and with 16S-proven L. crispatus (nc) = 5), L. iners-dominated VMB cluster (ni = 11, nc = 4), moderate dysbiosis (ni = 12, nc = 2); and severe dysbiosis (ni = 8, nc = 2). The relative abundances of proteins that were considered specific for L. iners and L. crispatus were compared among VMB groups.Results
Forty Lactobacillus proteins were identified of which 7 were specific for L. iners and 11 for L. crispatus. The relative abundances of L. iners DNA starvation/stationary phase protection protein (DPS), and the glycolysis enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and glucose-6-phosphate isomerase (GPI), were significantly decreased in women with L. iners-containing dysbiosis compared to women with a L. iners-dominated VMB, independent of vaginal pH and L. iners abundance. Furthermore, L. iners DPS, GAPDH, GPI, and fructose-bisphosphate aldolase (ALDO) were significantly negatively associated with vaginal pH. Glycolysis enzymes of L. crispatus showed a similar negative, but nonsignificant, trend related to dysbiosis.Conclusions
Most identified Lactobacillus proteins had conserved intracellular functions, but their high abundance in CVL supernatant might imply an additional extracellular (moonlighting) role. Our findings suggest that these proteins can be important in maintaining a Lactobacillus-dominated VMB. Functional studies are needed to investigate their roles in vaginal bacterial communities and whether they can be used to prevent vaginal dysbiosis. 相似文献995.
Valerie C. Henderson Jonathan Kimmelman Dean Fergusson Jeremy M. Grimshaw Dan G. Hackam 《PLoS medicine》2013,10(7)
Background
The vast majority of medical interventions introduced into clinical development prove unsafe or ineffective. One prominent explanation for the dismal success rate is flawed preclinical research. We conducted a systematic review of preclinical research guidelines and organized recommendations according to the type of validity threat (internal, construct, or external) or programmatic research activity they primarily address.Methods and Findings
We searched MEDLINE, Google Scholar, Google, and the EQUATOR Network website for all preclinical guideline documents published up to April 9, 2013 that addressed the design and conduct of in vivo animal experiments aimed at supporting clinical translation. To be eligible, documents had to provide guidance on the design or execution of preclinical animal experiments and represent the aggregated consensus of four or more investigators. Data from included guidelines were independently extracted by two individuals for discrete recommendations on the design and implementation of preclinical efficacy studies. These recommendations were then organized according to the type of validity threat they addressed. A total of 2,029 citations were identified through our search strategy. From these, we identified 26 guidelines that met our eligibility criteria—most of which were directed at neurological or cerebrovascular drug development. Together, these guidelines offered 55 different recommendations. Some of the most common recommendations included performance of a power calculation to determine sample size, randomized treatment allocation, and characterization of disease phenotype in the animal model prior to experimentation.Conclusions
By identifying the most recurrent recommendations among preclinical guidelines, we provide a starting point for developing preclinical guidelines in other disease domains. We also provide a basis for the study and evaluation of preclinical research practice. Please see later in the article for the Editors'' Summary 相似文献996.
997.
998.
Elon Pras Nina Raben Eliahu Golomb Nadir Arber Ivona Aksentijevich Jonathan M. Schapiro Daniela Harel Giora Katz Uri Liberman Mordechai Pras Daniel L. Kastner 《American journal of human genetics》1995,56(6):1297-1303
Cystinuria is an autosomal recessive disease characterized by the development of kidney stones. Guided by the identification of the SLC3A1 amino acid–transport gene on chromosome 2, we recently established genetic linkage of cystinuria to chromosome 2p in 17 families, without evidence for locus heterogeneity. Other authors have independently identified missense mutations in SLC3A1 in cystinuria patients. In this report we describe four additional cystinuria-associated mutations in this gene: a frameshift, a deletion, a transversion inducing a critical amino acid change, and a nonsense mutation. The latter stop codon was found in all of eight Ashkenazi Jewish carrier chromosomes examined. This report brings the number of disease-associated mutations in this gene to 10. We also assess the frequency of these mutations in our 17 cystinuria families. 相似文献
999.
1000.
We describe a prothoracic leg tibial flexion reflex (PTFR) of the praying mantis Tenodera aridifolia sinensis which is initiated by tactile stimulation of the movable spines of the ventro-medial border of the femur. This flexion reflex may be responsible for the continuous grasping of a captured prey by the mantid. 相似文献