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   10068篇
  免费   699篇
  国内免费   2篇
  10769篇
  2023年   68篇
  2022年   155篇
  2021年   258篇
  2020年   161篇
  2019年   213篇
  2018年   236篇
  2017年   224篇
  2016年   351篇
  2015年   516篇
  2014年   584篇
  2013年   729篇
  2012年   825篇
  2011年   802篇
  2010年   556篇
  2009年   401篇
  2008年   606篇
  2007年   555篇
  2006年   585篇
  2005年   484篇
  2004年   474篇
  2003年   412篇
  2002年   387篇
  2001年   85篇
  2000年   60篇
  1999年   86篇
  1998年   83篇
  1997年   91篇
  1996年   67篇
  1995年   42篇
  1994年   56篇
  1993年   44篇
  1992年   38篇
  1991年   38篇
  1990年   29篇
  1989年   33篇
  1988年   32篇
  1987年   26篇
  1986年   25篇
  1985年   32篇
  1984年   30篇
  1983年   26篇
  1982年   35篇
  1981年   33篇
  1980年   30篇
  1979年   17篇
  1977年   17篇
  1976年   12篇
  1975年   13篇
  1974年   11篇
  1973年   18篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Journal of Mammalian Evolution - Late Pleistocene cave lions are one of the most iconic species of Northern Hemisphere Quaternary taphocoenoses. Despite their often-scarce record in cave...  相似文献   
62.

Background

Many studies have addressed the relationship between iron deficiency anemia (IDA) and cognitive impairment, but none have evaluated the role of non-iron deficiency anemia (NIDA). One of the main causes of NIDA in developing countries is AI, largely due to infectious diseases, whereby iron is shunted away from bio-available forms to storage forms, making it less accessible for use by host tissues. The objective of this study was to determine the effect of NIDA, due largely to AI in this context, on cognitive function after adjustment for potential confounders.

Methodology

This cross-sectional study was conducted in Leyte, The Philippines among 322 children ages 7–18 years. Blood samples were collected and analyzed at the time of cognition testing. Three stool samples were collected and evaluated by the Kato Katz method for quantitative assessment for Schistosoma japonicum and geo-helminth infection. Socio-economic status (SES) was evaluated by survey. Linear regression models were used to quantify the adjusted relationship between performance in different cognitive domains and both IDA and NIDA.

Principal Findings

After adjusting for age, sex, SES and nutritional status, children in the NIDA had lower scores on the PNIT (P = <0.05) and the WRAML memory domain (P<0.05) compared to children in the non-anemic group. Children in the IDA had lower performance on the PNIT compared to the non-anemic group after controlling for potential confounders (P<0.05).

Conclusions

NIDA, predominantly due to AI in this context, was related to lower performance on two tests of cognitive function. This is likely due to decreased delivery of iron to host tissues in this context, including the CNS.  相似文献   
63.
Deficiency of argininosuccinate lyase (ASL) causes argininosuccinic aciduria, an urea cycle defect that may present with a severe neonatal onset form or with a late onset phenotype. To date phenotype-genotype correlations are still not clear because biochemical assays of ASL activity correlate poorly with clinical severity in patients. We employed a yeast-based functional complementation assay to assess the pathogenicity of 12 missense ASL mutations, to establish genotype-phenotype correlations, and to screen for intragenic complementation. Rather than determining ASL enzyme activity directly, we have measured the growth rate in arginine-free medium of a yeast ASLnull strain transformed with individual mutant ASL alleles. Individual haploid strains were also mated to obtain diploid, “compound heterozygous” yeast. We show that the late onset phenotypes arise in patients because they harbor individual alleles retaining high residual enzymatic activity or because of intragenic complementation among different mutated alleles. In these cases complementation occurs because in the hybrid tetrameric enzyme at least one active site without mutations can be formed or because the differently mutated alleles can stabilize each other, resulting in partial recovery of enzymatic activity. Functional complementation in yeast is simple and reproducible and allows the analysis of large numbers of mutant alleles. Moreover, it can be easily adapted for the analysis of mutations in other genes involved in urea cycle disorders.Argininosuccinic aciduria (ASAuria, MIM 207900)3 is an autosomal recessive disorder of the urea cycle caused by mutations of the ASL gene (hASL, MIM 608310), encoding argininosuccinate lyase (ASL; EC 4.3.2.1.) (1). This enzyme is ubiquitously expressed and catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate. ASL belongs to a superfamily of hydrolases that includes adenylosuccinate lyase and fumarase, which share a homotetrameric structure and a similar catalytic mechanism. The tetrameric structure of ASL accounts for the phenomenon of intragenic complementation. This particular situation occurs when a multimeric protein is formed from subunits produced by differently mutated alleles of the same gene. On complementation, a partially functional hybrid protein is produced from the two distinct types of mutant subunits, neither of which individually has appreciable enzymatic activity (2).ASL participates to the urea cycle, and in humans it is essential for ammonia detoxification, whereas in lower organisms it is required for the biosynthesis of arginine. Saccharomyces cerevisiae strains harboring a deletion of the homolog of human ASL (ARG4) cannot grow on media lacking arginine (3).ASAuria is characterized by accumulation of argininosuccinic acid (ASA) in body fluids, and severe hyperammonaemia. The disease displays clinical heterogeneity with two main clinical phenotypes: the acute/neonatal onset form, with symptoms rapidly progressing to deep coma, apnea, and death (1), and the subacute/late onset type, which is diagnosed in infancy or childhood (4). Such patients may present simply with mental retardation or an epileptic disorder. In both types the diagnosis is established unambiguously by measuring plasma levels of ammonia (not always elevated in the late onset form), ASA, and its anhydrides by plasma amino acids assay (1). Over 40 mutations of the ASL gene have been reported, both amino acid substitutions and truncating variants, which are scattered throughout the gene (5, 6).We have previously reported the identification of novel mutations of the ASL gene in a cohort of Italian patients (7). In this study we employed a yeast model to validate the pathogenicity of missense ASL mutations found in our cohort, to study the effects of different allelic combinations, and to establish possible genotype-phenotype correlations.  相似文献   
64.
65.
Eighteen green turtles Chelonia mydas recovered from the Atlantic and Gulf coasts of Florida and Tortuguero National Park, Costa Rica, were diagnosed with renal oxalosis by histopathological examination. Affected sea turtles included 14 adults and 4 immature animals, which comprised 26% (18/69) of green turtle necropsy cases available for review. Calcium oxalate deposition ranged from small to moderate amounts and was associated with granuloma formation and destruction of renal tubules. All affected turtles died from traumatic events or health problems unrelated to renal oxalosis; however, 1 immature turtle had notable associated renal injury. Crystal composition was confirmed by infrared and scanning electron microscopy and energy dispersive X-ray analysis. The source of calcium oxalate is unknown and is presumed to be of dietary origin.  相似文献   
66.
Rajchenberg M 《Mycologia》2011,103(4):677-702
This is a review of the available knowledge on nuclear behavior of the mycelium within polypore genera (Agaricomycetes, Basidiomycota). Information on 68 genera showed that nuclear behavior is a distinct and consistent feature at genus level and can be coupled with phylogenetic differentiation. The sole exception was found in Polyporus, where different species with normal, heterocytic and astatocoenocytic nuclear behaviors were found. Of the 68 genera treated 41 (60.3%) displayed a normal nuclear behavior, nine (13.2%) were heterocytic, nine (13.2%) were astatocoenocytic and another eight (11.8%) were holocoenocytic. In 95% of the genera a unique compatibility system was found, with the exceptions of Antrodia, which includes both homothallic and bipolar species all associated with a normal nuclear behavior, and Spongipellis, in which bipolar and tetrapolar species are found, all displaying an astatocoenocytic nuclear behavior. Normal and heterocytic nuclear behaviors were associated mostly with tetrapolarity, astatocoenocity was associated mostly with bipolarity, and holocoenocity was associated with either bipolarity or purported homothallism. The combination of nuclear behavior with mating system and brown or white rot capability appeared as a strong feature characterizing and distinguishing the genera of polypores, each combination being valuable to differentiate between apparently related genera, as is supported by phylogenetic studies. Several examples are presented to support this idea, as well as the cases of species that are problematic to this concept. Poroid genera of Hymenochaetaceae were treated apart because of the lack of knowledge regarding their nuclear behavior. In addition new information on the sexuality and/or nuclear behavior of 15 polyporoid taxa is given.  相似文献   
67.
68.
SUMMARY: ESS++ is a C++ implementation of a fully Bayesian variable selection approach for single and multiple response linear regression. ESS++ works well both when the number of observations is larger than the number of predictors and in the 'large p, small n' case. In the current version, ESS++ can handle several hundred observations, thousands of predictors and a few responses simultaneously. The core engine of ESS++ for the selection of relevant predictors is based on Evolutionary Monte Carlo. Our implementation is open source, allowing community-based alterations and improvements. AVAILABILITY: C++ source code and documentation including compilation instructions are available under GNU licence at http://bgx.org.uk/software/ESS.html.  相似文献   
69.
70.
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