全文获取类型
收费全文 | 777546篇 |
免费 | 92467篇 |
国内免费 | 909篇 |
专业分类
870922篇 |
出版年
2018年 | 6940篇 |
2016年 | 9370篇 |
2015年 | 13388篇 |
2014年 | 15602篇 |
2013年 | 21834篇 |
2012年 | 24977篇 |
2011年 | 25720篇 |
2010年 | 17190篇 |
2009年 | 15858篇 |
2008年 | 22671篇 |
2007年 | 23342篇 |
2006年 | 21862篇 |
2005年 | 21006篇 |
2004年 | 20768篇 |
2003年 | 19930篇 |
2002年 | 19317篇 |
2001年 | 33627篇 |
2000年 | 33537篇 |
1999年 | 26753篇 |
1998年 | 10013篇 |
1997年 | 10356篇 |
1996年 | 9864篇 |
1995年 | 9306篇 |
1994年 | 9007篇 |
1993年 | 9171篇 |
1992年 | 22596篇 |
1991年 | 22446篇 |
1990年 | 21751篇 |
1989年 | 21208篇 |
1988年 | 19524篇 |
1987年 | 18756篇 |
1986年 | 17634篇 |
1985年 | 17622篇 |
1984年 | 14716篇 |
1983年 | 12746篇 |
1982年 | 9737篇 |
1981年 | 8802篇 |
1980年 | 8324篇 |
1979年 | 13880篇 |
1978年 | 11246篇 |
1977年 | 10095篇 |
1976年 | 9545篇 |
1975年 | 10740篇 |
1974年 | 11490篇 |
1973年 | 11293篇 |
1972年 | 10136篇 |
1971年 | 9367篇 |
1970年 | 7852篇 |
1969年 | 7660篇 |
1968年 | 6951篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
Comparison of statistics for candidate-gene association studies using cases and parents. 总被引:17,自引:5,他引:12 下载免费PDF全文
Studies of association between candidate genes and disease can be designed to use cases with disease, and in place of nonrelated controls, their parents. The advantage of this design is the elimination of spurious differences due to ethnic differences between cases and nonrelated controls. However, several statistical methods of analysis have been proposed in the literature, and the choice of analysis is not always clear. We review some of the statistical methods currently developed and present two new statistical methods aimed at specific genetic hypotheses of dominance and recessivity of the candidate gene. These new methods can be more powerful than other current methods, as demonstrated by simulations. The basis of these new statistical methods is a likelihood approach. The advantage of the likelihood framework is that regression models can be developed to assess genotype-environment interactions, as well as the relative contribution that alleles at the candidate-gene locus make to the relative risk (RR) of disease. This latter development allows testing of (1) whether interactions between alleles exist, on the scale of log RR, and (2) whether alleles originating from the mother or father of a case impart different risks, i.e., genomic imprinting. 相似文献
44.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
45.
U Mura A M Osman A S Mohamed D Di Martino P L Ipata 《Comparative biochemistry and physiology. B, Comparative biochemistry》1987,87(1):157-160
The preservation of purine ring as purine bases appears to be a common feature of camel liver. Hepatic guanine appears to be actively converted into GMP in the camel rather than further degraded. The limiting step of guanine degradation appears to be the lack of hepatic guanase activity. Higher purine bases over uric acid ratios were found in camel urine with respect to those of zebu. 相似文献
46.
47.
48.
49.
Ernst Wm. Spannhake Terry L. Adams Steven R. Kleeberger 《Prostaglandins & other lipid mediators》1985,30(6):1041-1055
The effects of repeated antigen exposure on the synthesis of mediators by lung tissues are not well understood. To investigate the influence of antigen challenge on the synthesis of prostaglandins by central airway and peripheral lung tissues, fourteen sensitive sheep underwent biweekly exposure to aerosolized antigen (7) or saline (7). Following the fifth exposure, microsomal and high speed supernatant fractions were prepared from trachealis muscle and lung parenchyma. Synthesis of thromboxane (TX) A2, prostaglandin (PG) D2 and PGI2 from the PG endoperoxide intermediate, PGH2, was assayed over a range of substrate concentrations from 3–200 uM. Synthesis of PGI2 by trachealis microsomes was approximately 5-fold greater than that of TXA2. PGI2 and TXA2 production was identical in tracheal preparations from Ascaris- and saline-exposed animals. In parenchymal tissues, where TXA2 production predominated over PGI2 by 9-fold, preparations from Ascaris- exposed animals synthesized 50% more TXA2 than controls at PGH2 concentrations of 25 uM and above, whereas synthesis of PGI2 and PGD2 were similar in preparations from both groups of animals. The density of pulmonary mast cells was decreased by 21% in the Ascaris group, whereas polymorphonuclear leukocyte density was unchanged. These results demonstrate the differential synthesis of TXA2 and PGI2 in central airways and peripheral lung regions of the sheep. They further indicate that repeated exposure of the airways to antigen selectively enhances TXA2 synthesis in the lung periphery of sensitized animals. The site of this increased enzymatic activity, whether in resident cells or newly-infiltrated cells, has not been determined. 相似文献
50.
F F Dias M K Shaikh Y B Bhatt D C Modi V R Subramanyam 《The Journal of applied bacteriology》1986,60(4):271-275
Mutants of Bacillus amyloliquefaciens resistant to at least 10 micrograms/ml of tunicamycin were isolated and shown to be pleiotropic. The mutants were more resistant to streptomycin, chloramphenicol, kanamycin and neomycin than was the parent strain but less resistant to penicillin G and tetracycline. They were more autolytic, presumably due to an altered cell wall. The mutants produced reduced levels of amylase, penicillinase and both metal and serine protease besides having an enhanced sporulation frequency and being more motile. 相似文献