全文获取类型
收费全文 | 12572篇 |
免费 | 936篇 |
国内免费 | 6篇 |
出版年
2022年 | 66篇 |
2021年 | 112篇 |
2020年 | 70篇 |
2019年 | 110篇 |
2018年 | 120篇 |
2017年 | 151篇 |
2016年 | 208篇 |
2015年 | 268篇 |
2014年 | 364篇 |
2013年 | 695篇 |
2012年 | 601篇 |
2011年 | 610篇 |
2010年 | 400篇 |
2009年 | 374篇 |
2008年 | 602篇 |
2007年 | 594篇 |
2006年 | 539篇 |
2005年 | 540篇 |
2004年 | 538篇 |
2003年 | 550篇 |
2002年 | 498篇 |
2001年 | 476篇 |
2000年 | 495篇 |
1999年 | 389篇 |
1998年 | 152篇 |
1997年 | 149篇 |
1996年 | 122篇 |
1995年 | 132篇 |
1994年 | 123篇 |
1993年 | 106篇 |
1992年 | 299篇 |
1991年 | 260篇 |
1990年 | 255篇 |
1989年 | 247篇 |
1988年 | 252篇 |
1987年 | 205篇 |
1986年 | 213篇 |
1985年 | 187篇 |
1984年 | 129篇 |
1983年 | 148篇 |
1982年 | 119篇 |
1981年 | 83篇 |
1980年 | 80篇 |
1979年 | 138篇 |
1978年 | 79篇 |
1977年 | 77篇 |
1975年 | 60篇 |
1973年 | 65篇 |
1972年 | 58篇 |
1970年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
951.
952.
Human papillomavirus (HPV), particularly HPV16, is strongly associated with premalignant lesions of the uterine cervix and cervical cancer. However, HPV infection is a common sexually transmitted disease and only a few women develop cervical cancer. Although the presence of HPV and abnormal cytology are independent risk factors for cervical diseases, implementing both tests on every woman is argued not to be recommended mainly in terms of cost-effectiveness. During a 20-month period between October, 1994, and May, 1996, cervical swabs from 207 women who were referred for colposcopy because of cervical dyskaryosis (Papanicolaou class IIIa or higher) were examined by PCR for the presence of HPV16. When these women were divided into two groups; i.e., group A consisting of women who were 44 years old or younger (n = 111), and group B consisting of women who were 45 years old or older (n = 96), the risk of having premalignant and malignant cervical diseases upon infection with HPV16 was approximately 8 times higher in group A than in group B. Thus, we conclude that HPV-testing should be implemented on every young woman with an abnormal Papanicolaou smear test. 相似文献
953.
INTRODUCTION: CA19-9 is one of the most important tumor markers used in patients with colorectal cancer, mainly in radical surgery follow-up. AIM: The purpose of this study was to evaluate the preoperative CA19-9 level obtained from a peripheral vein (PV) and compare it to the level obtained from the mesenteric vein (MV). MATERIALS AND METHODS: Blood was collected from a PV of the arm and from the MV of 59 patients with colorectal cancer before primary surgery. Of these 59 patients fourteen had stage I disease, 10 stage II, 22 stage III, and 13 stage IV. CA19-9 was determined in serum by immunoenzymatic assay (Abbott Diagnostica). RESULTS: Fifteen patients (24%) had elevated serum levels of CA19-9 in the MV and 13 (22%) in the PV. None of the stage I or II patients had elevated serum levels of CA19-9. There were no differences between marker levels in blood collected from the MV or PV, independent of clinical stage. The CA19-9 values obtained from the MV differed significantly in the different stages of the disease according to the Kruskal-Wallis analysis (p=0.026); this difference was not statistically significant (p=0.08) in serum from the PV. There was no correlation between venous infiltration by the tumor and positivity of CA19-9 serum levels collected from the mesenteric vein. We observed a close correlation between the serum levels of CA19-9 collected from the PV and from the MV (r=0.9). CONCLUSION: The current study demonstrates a close correlation between the serum levels of CA19-9 collected from a peripheral vein and from the mesenteric vein. Our results confirmed the poor sensitivity of serum CA19-9 at diagnosis, independent of the collection site. 相似文献
954.
The presynaptic particle web: ultrastructure, composition, dissolution, and reconstitution 总被引:26,自引:0,他引:26
Phillips GR Huang JK Wang Y Tanaka H Shapiro L Zhang W Shan WS Arndt K Frank M Gordon RE Gawinowicz MA Zhao Y Colman DR 《Neuron》2001,32(1):63-77
We report the purification of a presynaptic "particle web" consisting of approximately 50 nm pyramidally shaped particles interconnected by approximately 100 nm spaced fibrils. This is the "presynaptic grid" described in early EM studies. It is completely soluble above pH 8, but reconstitutes after dialysis against pH 6. Interestingly, reconstituted particles orient and bind PSDs asymmetrically. Mass spectrometry of purified web components reveals major proteins involved in the exocytosis of synaptic vesicles and in membrane retrieval. Our data support the idea that the CNS synaptic junction is organized by transmembrane adhesion molecules interlinked in the synaptic cleft, connected via their intracytoplasmic domains to the presynaptic web on one side and to the postsynaptic density on the other. The CNS synaptic junction may therefore be conceptualized as a complicated macromolecular scaffold that isostatically bridges two closely aligned plasma membranes. 相似文献
955.
In order to measure the circular dichroism (CD) spectrum in the solid state of a chiral compound which has no chromophore, an induced CD spectral method was developed by measuring the spectrum of the inclusion crystal of the chiral compound with a simple achiral aromatic host compound in the Nujol mulls. 相似文献
956.
Tokunaga M Shiraishi Y Odachi M Mizukami M Tokunaga H Philo JS Arakawa T Ishibashi M Tanaka R Takagi H 《Bioscience, biotechnology, and biochemistry》2001,65(6):1379-1387
The groESL locus of a protein-hypersecreting bacterium, Bacillus brevis, was cloned by PCR using primers designed based on the DNA sequence of a B. subtilis homolog. GroEL protein was purified to apparent homogeneity and its ATPase activity was characterized: it hydrolyzed ATP, CTP, and TTP in this order of reaction rate, and its specific activity for ATP was 0.1 micromole/min/mg protein. Purified GroEL forms a tetradecamer. GroEL was estimated to contain 22% alpha-helix, 24% beta-sheet, and 19% turn structures, by CD measurement. GroES protein was also highly purified to examine its chaperonin activity. GroEL protected from thermal inactivation of and showed refolding-promoting activity for malate dehydrogenase, strictly depending on the presence of ATP and GroES. 相似文献
957.
In this study, a highly active foliar aminopeptidase preferentially releasing N-terminal alanine from artificial substrates was purified and characterized from cucumber (Cucumis sativus L. suyo). The enzyme had a molecular mass of 200 kDa consisting of two subunits of 95 kDa. It was a metalloprotease the pH optimum of which was 8 to 9. It cleaved Ala-, Gly-, Met-, Ser-, Leu-, Lys-, and Arg artificial substrates. An internal amino acid sequence was similar to those of aminopeptidase N (clan MA, family M1) of microorganisms, and was very similar to that of a putative aminopeptidase N of Arabidopsis thaliana. From these results, the highly active aminopeptidase in cucumber leaves was identified to be a plant aminopepitdase N. 相似文献
958.
Role of cadherins in maintaining the compartment boundary between the cortex and striatum during development 总被引:4,自引:0,他引:4
Inoue T Tanaka T Takeichi M Chisaka O Nakamura S Osumi N 《Development (Cambridge, England)》2001,128(4):561-569
In ventricular cells of the mouse telencephalon, differential expression of cadherin cell adhesion molecules defines neighbouring regions; R-cadherin delineates the future cerebral cortex, while cadherin-6 delineates the lateral ganglionic eminence. By using cell labelling analyses in the whole embryo culture system, we demonstrated that the interface between R-cadherin and cadherin-6 expression is a boundary for cell lineage restriction at embryonic day 10.5. Interestingly, when a group of cells with exogenous cadherin-6 were generated to straddle the cortico-straital boundary by electroporation at embryonic day 11.0, ectopic cadherin-6-expressing cortical cells were sorted into the striatal compartment, and the reverse was the trend for ectopic R-cadherin-expressing striatal cells. Although cadherin-6 gene knockout mice engineered in this study showed no obvious phenotype in telencephalic compartmentalisation, the preferential sorting of ectopic cadherin-6-expressing cells was abolished in this mutant background. Thus, the differential expression pattern of cadherins in the embryonic telencephalon is responsible for maintaining the cortico-striatal compartment boundary. 相似文献
959.
A study of the ontogeny of the lateral line system in leptocephali of the Japanese eel Anguilla japonica reveals the existence of three morphologically different types of lateral line organs. Type I is a novel sensory organ with hair cells bearing a single kinocilium, lacking stereocilia, distributed mainly on the head of larvae, and morphologically different from typical superficial neuromasts of the lateral line system. Its developmental sequence suggests that it may be a presumptive canal neuromast. Type II is an ordinary superficial neuromast, common in other teleost larvae, which includes presumptive canal neuromasts that first appear on the trunk and accessory superficial neuromasts that later appear on the head and trunk. Type III is a very unusual neuromast located just behind the orbit, close to the otic vesicle, with radially oriented hair cells, suggesting that these serve as multiple axes of sensitivity for mechanical stimuli. The behavior of larval eels suggests that the radially oriented neuromasts may act as the sole mechanosensory organ until the ordinary superficial neuromasts develop. The finding that larval eels possess a well-developed mechanosensory system suggests the possibility that they are also capable of perceiving weak environmental mechanical stimuli, like other teleost larvae. 相似文献
960.
The putative Rhodococcus rrn promoter region was cloned from the benzothiophene desulfurizing Rhodococcus sp. strain T09, and the dibenzothiophene desulfurizing gene, dsz, was expressed under the control of the putative rrn promoter in the strain T09 using a Rhodococcus–E.coli shuttle vector. Strain T09 harboring the expression vector, pNT, could desulfurize dibenzothiophene in the presence of inorganic
sulfate, methionine, or cysteine, while the Dsz phenotype was completely repressed in recombinant cells carrying the gene
under the control of the native dsz promoter under the same conditions. Among the sulfur sources examined, no intermediates were detected and only the final
desulfurized product, 2-hydroxy-biphenyl, was produced using ammonium sulfate as the sulfur source.
Received: 4 December 2001 / Accepted: 7 January 2002 相似文献