全文获取类型
收费全文 | 84754篇 |
免费 | 7669篇 |
国内免费 | 42篇 |
出版年
2023年 | 237篇 |
2022年 | 191篇 |
2021年 | 1453篇 |
2020年 | 879篇 |
2019年 | 1108篇 |
2018年 | 1439篇 |
2017年 | 1241篇 |
2016年 | 2136篇 |
2015年 | 3630篇 |
2014年 | 4047篇 |
2013年 | 4811篇 |
2012年 | 6536篇 |
2011年 | 6481篇 |
2010年 | 4170篇 |
2009年 | 3798篇 |
2008年 | 5428篇 |
2007年 | 5431篇 |
2006年 | 5233篇 |
2005年 | 5031篇 |
2004年 | 4891篇 |
2003年 | 4667篇 |
2002年 | 4401篇 |
2001年 | 857篇 |
2000年 | 622篇 |
1999年 | 903篇 |
1998年 | 1136篇 |
1997年 | 744篇 |
1996年 | 667篇 |
1995年 | 644篇 |
1994年 | 615篇 |
1993年 | 666篇 |
1992年 | 552篇 |
1991年 | 517篇 |
1990年 | 459篇 |
1989年 | 395篇 |
1988年 | 420篇 |
1987年 | 340篇 |
1986年 | 317篇 |
1985年 | 399篇 |
1984年 | 514篇 |
1983年 | 400篇 |
1982年 | 501篇 |
1981年 | 485篇 |
1980年 | 414篇 |
1979年 | 295篇 |
1978年 | 320篇 |
1977年 | 280篇 |
1976年 | 261篇 |
1975年 | 202篇 |
1974年 | 241篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Kimberly R. Holloway Vidya C. Sinha Michael J. Toneff Wen Bu Susan G. Hilsenbeck Yi Li 《PloS one》2015,10(1)
While most breast cancers are thought to arise from the luminal layer of the breast tissue, it remains unclear which specific cells in the luminal layer are the cells of origin of breast cancer. We have previously reported that WAP-positive luminal epithelial cells are at increased susceptibility to tumor initiation by ErbB2 compared to the bulk population, while the mammary cells with canonical Wnt signaling activity fail to evolve into tumors upon ErbB2 activation. Here, we used retrovirus to introduce ErbB2 into the Krt6a-positive mammary progenitor subset of the luminal epithelium and, for comparison, into the mammary luminal epithelium indiscriminately. Tumors developed from both groups of cells with a similar latency. These data indicate that the Krt6a-positive subset of mammary epithelial cells can be induced to form cancer by ErbB2 but it is not more susceptible to tumorigenesis initiated by ErbB2 than the bulk population of the luminal epithelium. 相似文献
32.
Dimitrios Moustakas Michael Mezzio Branden R. Rodriguez Mic Andre Constable Margaret E. Mulligan Evelyn B. Voura 《PloS one》2015,10(4)
The stimulant effect of energy drinks is primarily attributed to the caffeine they contain. Many energy drinks also contain other ingredients that might enhance the tonic effects of these caffeinated beverages. One of these additives is guarana. Guarana is a climbing plant native to the Amazon whose seeds contain approximately four times the amount of caffeine found in coffee beans. The mix of other natural chemicals contained in guarana seeds is thought to heighten the stimulant effects of guarana over caffeine alone. Yet, despite the growing use of guarana as an additive in energy drinks, and a burgeoning market for it as a nutritional supplement, the science examining guarana and how it affects other dietary ingredients is lacking. To appreciate the stimulant effects of guarana and other natural products, a straightforward model to investigate their physiological properties is needed. The planarian provides such a system. The locomotor activity and convulsive response of planarians with substance exposure has been shown to provide an excellent system to measure the effects of drug stimulation, addiction and withdrawal. To gauge the stimulant effects of guarana we studied how it altered the locomotor activity of the planarian species Dugesia tigrina. We report evidence that guarana seeds provide additional stimulation over caffeine alone, and document the changes to this stimulation in the context of both caffeine and glucose. 相似文献
33.
34.
People are biased partisans: they tend to agree with policies from political parties they identify with, independent of policy content. Here, we investigate how physiological reactions to political parties shape bias. Using changes in galvanic skin conductance responses to the visual presentation of party logos, we obtained an implicit and physiological measure of the affective arousal associated with political parties. Subsequently, we exposed subjects to classical party cue experiments where the party sponsors of specific policies were experimentally varied. We found that partisan bias only obtains among those exhibiting a strong physiological reaction to the party source; being a self-reported party identifier is not sufficient on its own. This suggests that partisan bias is rooted in implicit, affective reactions. 相似文献
35.
Most pathogen detection tests are imperfect, with a sensitivity < 100%, thereby resulting in the potential for a false negative, where a pathogen is present but not detected. False negatives in a sample inflate the number of non-detections, negatively biasing estimates of pathogen prevalence. Histological examination of tissues as a diagnostic test can be advantageous as multiple pathogens can be examined and providing important information on associated pathological changes to the host. However, it is usually less sensitive than molecular or microbiological tests for specific pathogens. Our study objectives were to 1) develop a hierarchical occupancy model to examine pathogen prevalence in spring Chinook salmon Oncorhynchus tshawytscha and their distribution among host tissues 2) use the model to estimate pathogen-specific test sensitivities and infection rates, and 3) illustrate the effect of using replicate within host sampling on sample sizes required to detect a pathogen. We examined histological sections of replicate tissue samples from spring Chinook salmon O. tshawytscha collected after spawning for common pathogens seen in this population: Apophallus/echinostome metacercariae, Parvicapsula minibicornis, Nanophyetus salmincola/ metacercariae, and Renibacterium salmoninarum. A hierarchical occupancy model was developed to estimate pathogen and tissue-specific test sensitivities and unbiased estimation of host- and organ-level infection rates. Model estimated sensitivities and host- and organ-level infections rates varied among pathogens and model estimated infection rate was higher than prevalence unadjusted for test sensitivity, confirming that prevalence unadjusted for test sensitivity was negatively biased. The modeling approach provided an analytical approach for using hierarchically structured pathogen detection data from lower sensitivity diagnostic tests, such as histology, to obtain unbiased pathogen prevalence estimates with associated uncertainties. Accounting for test sensitivity using within host replicate samples also required fewer individual fish to be sampled. This approach is useful for evaluating pathogen or microbe community dynamics when test sensitivity is <100%. 相似文献
36.
37.
38.
Shigeaki Nonoyama Mitsunobu Shimadzu Hano Toru Kuniaki Seyama Hiroyuki Nunoi Michael Neubauer Jun-ichi Yata Hans D. Ochs 《Human genetics》1997,99(5):624-627
X-linked hyper-IgM syndrome (XHIM) is a rare primary immunodeficiency caused by a defective CD40 ligand. We identified mutations
of the CD40 ligand gene in 13 unrelated Japanese XHIM patients. Of the four patients with missense mutations, one had a mutation
within the transmembrane domain, and the three others had mutations affecting the TNF homology region of the extracellular
domain. Two of the missense mutations resulted in the substitution of amino acids that are highly conserved in TNF family
proteins. Three patients had nonsense mutations, all of which resulted in the truncation of the TNF homology domain of the
CD40 ligand. Three patients had genomic DNA deletions of 2, 3 or 4 nucleotides, respectively. All of the deletions were flanked
by direct repeat sequences, suggesting that these deletions were caused by slipped mispairing. Three patients had mutations
within introns resulting in altered splicing, and multiple splicing products were found in one patient. Thus, each of the
13 Japanese patients had different mutations, 9 of them being novel mutations. These results indicate that mutations in XHIM
are highly heterogeneous, although codon 140 seems to be a hot spot of the CD40 ligand gene since two additional point mutations
were located at Trp 140, bringing the total numbers of mutations affecting codon 140 to six. In one XHIM family with a missense
mutation, prenatal diagnosis was performed by single-strand conformation polymorphism analysis of genomic DNA of a male fetus.
Received: 20 August 1996 相似文献
39.
40.
Risks and benefit evaluation for controlled human infection studies, where healthy volunteers are deliberately exposed to infectious agents to evaluate vaccine efficacy, should be explicit, systematic, thorough, and non-arbitrary. Decision analysis promotes these qualities using four steps: (1) determining explicit criteria and measures for evaluation, (2) identifying alternatives to the study, (3) defining the models used to estimate the measures for each alternative, and (4) running the models to produce the estimates and compare the alternatives. In this paper, we describe how decision analysis might be applied by funders and regulators, as well as by others contemplating the use of novel controlled human infection studies for vaccine development and evaluation. 相似文献