全文获取类型
收费全文 | 49712篇 |
免费 | 4546篇 |
国内免费 | 30篇 |
出版年
2022年 | 201篇 |
2021年 | 758篇 |
2020年 | 452篇 |
2019年 | 567篇 |
2018年 | 701篇 |
2017年 | 647篇 |
2016年 | 1039篇 |
2015年 | 1751篇 |
2014年 | 1934篇 |
2013年 | 2543篇 |
2012年 | 3217篇 |
2011年 | 3277篇 |
2010年 | 2126篇 |
2009年 | 1903篇 |
2008年 | 2705篇 |
2007年 | 2812篇 |
2006年 | 2742篇 |
2005年 | 2627篇 |
2004年 | 2527篇 |
2003年 | 2399篇 |
2002年 | 2298篇 |
2001年 | 538篇 |
2000年 | 444篇 |
1999年 | 607篇 |
1998年 | 729篇 |
1997年 | 547篇 |
1996年 | 469篇 |
1995年 | 427篇 |
1994年 | 411篇 |
1993年 | 440篇 |
1992年 | 475篇 |
1991年 | 411篇 |
1990年 | 402篇 |
1989年 | 389篇 |
1988年 | 364篇 |
1987年 | 326篇 |
1986年 | 350篇 |
1985年 | 375篇 |
1984年 | 453篇 |
1983年 | 372篇 |
1982年 | 477篇 |
1981年 | 490篇 |
1980年 | 384篇 |
1979年 | 318篇 |
1978年 | 332篇 |
1977年 | 293篇 |
1976年 | 326篇 |
1975年 | 220篇 |
1974年 | 274篇 |
1973年 | 271篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
Joseph A. Ezzo Clark Spencer Larsen James H. Burton 《American journal of physical anthropology》1995,98(4):471-481
Multielement analysis was performed on bone samples extracted from the femora of 39 adults from three mortuary sites (Johns Mound, Santa Catalina de Guale, and Santa Catalina de Guale de Santa Meria) and time periods (late preagricultural, early contact, and late contact) in the Georgia Bight. This study was used to investigate whether elemental analysis would support or contradict other lines of data regarding diets and dietary change previously generated for the region. The data are in agreement with an earlier interpretation, based on stable isotopes, that dietary maize increases through time but fails to support the idea that marine resources decreased in importance. Rather, it appears that the wild plant food component of the diets decreases as maize increases in importance; throughout the sequence, marine resources comprise a significant portion of the diets. © 1995 Wiley-Liss, Inc. 相似文献
972.
973.
Shunji Tomatsu Seiji Fukuda Alan Cooper James E. Wraith Atsushi Uchiyama Toshinori Hori Yoshinori Nakashima Naoto Yamada Kazuko Sukegawa Naomi Kondo Yasuyuki Suzuki Nobuyuki Shimozawa Tadao Orii 《Human genetics》1995,95(4):376-381
Ninety-six alleles (36 alleles of Japanese and 60 of Caucasian origin) from forty-eight patients with mucopolysaccharidosis IVA were investigated for structural gene alterations using Southern blot analysis. All patients had a previously demonstrated deficiency of N-acetylgalactosamine-6-sulfate-sulfatase and exhibited a wide spectrum of clinical severity. Initially, using the fulllength cDNA as a probe, five of 36 chromosomes from the Japanese patients revealed similar rearrangements with respect to DNA digested with BamHI, SacI, and XhoI. Subsequent analysis using seven genomic fragments, covering the entire gene, enhanced the detection of aberrant fragments produced by the above restriction enzymes. Conversely, the 60 chromosomes of Caucasian origin revealed no evidence of large structural rearrangements when analyzed by these methods. There was a statistically significant difference between the two populations (P < 0.01). A severely affected Japanese patient showed structural rearrangements on both chromosomes by means of BamHI blots. An 8.0-kb fragment and a highly polymorphic 7.0-kb to 11.0-kb fragment present in normal individuals disappeared and two aberrant fragments of 11.5 kb and 12.0 kb were observed. Three other Japanese patients also showed these two aberrant fragments, in addition to the normal fragment pattern, and were thus heterozygous for this rearrangement. Interpretation of Southern blots was difficult because of the complexity of polymorphic bands resulting from variable number of tandem repeat elements. However, by utilizing these aberrant fragments or polymorphic bands, carrier detection was effective, even in families with poorly characterized mutations. Hybridization with probe MG-A (5end genomic probe in intron 1) showed a 8.4-kb fragment in BamHI blots of one Japanese and one Caucasian patient; XhoI, SacI, and EcoRI blots were normal. Since this BamHI alteration was also observed in one normal control, it appears to be a rare nonpathological polymorphism. 相似文献
974.
Andrew M. Jackson Anton B. Alexandrov S. Prescott Keith James 《Cancer immunology, immunotherapy : CII》1995,40(2):119-124
Intravesical immunotherapy for bladder cancer is the most effective form of tumour immunotherapy. Following repeated instillations
of bacillus Calmette-Guérin (BCG) organisms into the bladder large 0quantities of several cytokines are detected in the urine.
These cytokines include interleukins IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, tumour necrosis factor α (TNFα), interferon γ (IFNγ)
and also soluble intercellular adhesion molecule ICAM-1. In the work reported here we simultaneously quantified urinary levels
of TNFα, TNFβ, TNF receptor I and TNF receptor II by enzyme-linked immunosorbent assay (ELISA) techniques and compared this
with bioactive levels of TNF. This was undertaken with a limited number of patients throughout a course of six instillations
of immuno therapy. Sequential instillations of BCG induced secretion of TNFα and TNFβ into urine. These cytokines were not
always secreted simultaneously, perhaps suggesting differential regulation of their synthesis. Maximal concentrations of TNFα
were 675 pg/ml and TNFβ 47 pg/ml. High levels of both species of soluble TNF receptor were readily identified in urine. Maximal
levels of sTNF-RI were 6200 pg/ml (range from 0) and for sTNF-RII 7800 pg/ml (range from 0). Contrasting with earlier published
observations concerning cytokine levels, the concentration of soluble receptor did not increase with repeated instillation.
In apparent contrast with the ELISA data, very low levels of bioactive TNF were identified by the L929 bioassay (maximum concentration
1 U/ml) despite the elevated concen t ration of immunoreactive TNF. The large concentrations of soluble TNF receptor in patients’
urine samples could account for the apparently low bioactivity as determined by the L929 cytotoxicity assay. The precise nature
of the role of TNF in BCG immunotherapy remains undetermined; however, it is thought that proinflammatory cytokines are in
part responsible for the clinical efficacy of this therapeutic approach. Whether other cytokines are antogonised by soluble
binding proteins remains to be determined. Furthermore, whether TNF is bioactive in the bladder wall and only neutralised
in the urine also requires investigation.
Received: 24 August 1994 / Accepted: 17 October 1994 相似文献
975.
Construction and characterization of the chimeric monoclonal antibody E48 for therapy of head and neck cancer 总被引:1,自引:0,他引:1
Ruud H. Brakenhoff Frank B. van Gog James E. Looney Marijke van Walsum Gordon B. Snow Guus A. M. S. van Dongen 《Cancer immunology, immunotherapy : CII》1995,40(3):191-200
Data from an ongoing clinical radioimmunoscintigraphy trial indicate that99mTc-labeled monoclonal antibody (mAb) E48 is highly capable of selectively targeting squamous cell carcinoma of the head and neck (HNSCC). The percentage of the injected dose per gram of tumor tissue was found to be high, rendering mAbE48 a promising candidate mAb for therapeutic purposes. We now describe the construction of a chimeric (moouse/human) mAb E48 by recombinant DNA technology. The genes encoding the variable domains of the heavy and light chain were cloned and ligated into experession vectors containing the human 1 heavy-chain gene and the human k lightchain gene respectively. Biological properties of the resulting chimeric mAb E48 were compared to the murine form in vitro and in vivo. The reactivities of chimeric (c)mAb and murine (m)mAb E48 with HNSCC, as assessed by immunohistochemical staining as well as immuno-blotting were shown to be similar. The affinity constant appeared to be 0.9×1010 M–1 and 1.6×1010 M–1 for the mmAb and cmAb respectively. The biodistribution of both antibodies was tested by simultaneous injection into nude mice bearing human HNSCC xenografts. cmAb E48 was found to be cleared more rapidly from the blood than mmAb E48, resulting in a 30% lower tumor uptake but similar tumor to non-tumor ratios, 3 days after injection. Moreover, it was shown that cmAb E48 is highly capable of lysing HNSCC targets in ADCC assays in vitro, whereas the mmAb appeared to be almost incative. These data indicate that cmAb E48 has potential as a targeting agent for the eradication of HNSCC in man. 相似文献
976.
David A. Watson Vivek Kapur Daniel M. Musher James W. Jacobson James M. Musser 《Current microbiology》1995,31(4):251-259
This paper reports the cloning and sequencing of a region of DNA from Streptococcus pneumoniae serotype 3 surrounding transposon Tn916, insertion of which was previously shown to result in lack of expression of the extracellular capsule. Sequence analysis revealed that the transposon inserted into a consensus insertion site 71 bp from the 5 end of the cloned fragment. Within the clone, 3 downstream regions from two different pneumococcal lytA genes were identified, as well as a putative 194 AA open reading frame (ORF1). Moreover, two copies of the repeat element BOX, oriented in opposite directions, were located immediately 3 of orf1. Within the region bounded by the first pair of internal sequencing primers, analysis revealed that the fragment amplified by PCR was always of the same size. Moreover, Southern blotting showed that for all serotypes examined to date, homology exists with the cloned fragment. These results indicate that this region of the chromosome is highly conserved and, taken together with other independently derived data, suggest that interruptions or deletions within this DNA lead to unencapsulation. 相似文献
977.
Charles E. Sasser James G. Gosselink Erick M. Swenson D. Elaine Evers 《Wetlands Ecology and Management》1995,3(3):171-187
Floating marshes occur over 70% of the western Terrebonne Basin, Louisiana, USA, freshwater coastal wetlands. They are of several types: A free-floating thick-mat (45–60 cm) marsh dominated by Panicum hemitomon and Sagittaria lancifolia; a thick mat marsh dominated by Panicum hemitomon and Sagittaria lancifolia that floats part of the year, but whose vertical floating range is damped compared to adjacent water; and an irregularly-floating thin mat (< 30 cm) dominated by Eleocharis spp. in the spring and Ludwigia leptocarpa and Bidens laevis in the summer and fall. Floating mats must be almost entirely organic in order to be buoyant enough to float. The western Terrebonne wetlands receive large winter/spring supplies of suspended sediments from the Atchafalaya River. Even though sediment concentrations in the adjacent bayou are as high as 100 mg l–1, the Panicum hemitomon/Sagittaria lancifolia free-floating marsh probably receives no over-surface sediments since it floats continuously. The bulk density data of the damped-floating marsh, however, suggest some mineral sediment input, probably during winter when this marsh is submerged. These two types of floating marsh could not have developed in the present sediment regime of the Atchafalaya River, but as long as they remain floating can continue to exist. Thin floating mats are found in areas receiving the least sediment (<20 mg 1–1 suspended sediment concentration in adjacent bayous). This low sediment environment probably made possible their formation within the past 20 years. They may represent a transitional stage in mat succession from (1) existing thick-mat floating marsh to a degrading floating marsh, or (2) a floating marsh developing in shallow open water.Corresponding editor: D. Whigham 相似文献
978.
We studied the winter resource selection of muskoxen Ovibos moschatus in the High Arctic using a nested hierarchy of spatial scales 1) population range, 2) travel routes, 3) feeding sites (l e clusters of feeding craters), 4) feeding craters, and 5) diet (I e plant species) We found that, generally, patterns of selection remained consistent across all levels At successively smaller scales, muskoxen selected for higher graminoid abundance and particularly for thinner, softer snow cover, although we did not reject the hypothesis of random travel route selection Muskoxen uncovered forages from beneath the snow cover, by cratering, near the flonstic and nival extremes of availability Selection was consistently biased toward use of water sedge, Carex aquatilis As scale changed, however, muskoxen showed reversals of preference for some other forage species Diet was dominated by C aquatilis and cotton sedge, Eriophorum angustifolium , species characteristic of lowland meadows During spring melt, muskoxen moved to snow-free uplands to feed Dietary quality, as revealed by fecal nitrogen, increased at this time The consistency of the results across scales implied that these local levels of habitat selection occurred within one scaling domain 相似文献
979.
Mauseth James D. Uozumi Yoriko Plemons Brandon J. Landrum James V. 《Journal of plant research》1995,108(4):517-526
Wide-band tracheids are a specialized tracheid type in which an annular or helical secondary wall projects deeply into the
cell lumen. They are short, wide and spindle-shaped, and their bandlike secondary walls cover little of the primary wall,
leaving most of it available for water diffusion. Wide-band tracheids appear to store and conduct water while preventing the
spread of embolisms. They may be the most abundant tracheary element in the xylem, but they are always accompanied by at least
a few vessels. Typically, fibers are absent wherever wide-band tracheids are present. Wide-band tracheids occur in the primary
and secondary xylem of succulent stems, leaves and roots in genera of all three subfamilies of Cactaceae but were not found
in the relictual genusPereskia, which lacks succulent tissues. In the large subfamily Cactoideae, wide-band tracheids occur only in derived members, and
wide-band tracheids of North American Cactoideae are narrower and are aligned in a more orderly radial pattern than those
of South American Cactoideae. Wide-band tracheids probably arose at least three times in Cactaceae. 相似文献
980.
He Liu H. Peter Spielmann Nikolai B. Ulyanov David E. Wemmer Thomas L. James 《Journal of biomolecular NMR》1995,6(4):390-402
Summary The effect of experimental and integration errors on the calculations in interproton distances from NOE intensities is examined. It is shown that NOE intensity errors can have a large impact on the distances determined. When multiple spin (spin diffusion) effects are significant, the calculated distances are often underestimated, even when using a complete relaxation matrix analysis. In this case, the bias of distances to smaller values is due to the random errors in the NOE intensities. We show here that accurate upper and lower bounds of the distances can be obtained if the intensity errors are properly accounted for in the complete relaxation matrix calculations, specifically the MARDIGRAS algorithm. The basic MARDIGRAS algorithm has been previously described [Borgias, B.A. and James, T.L. (1990) J. Magn. Reson., 87, 475–487]. It has been shown to provide reasonably good interproton distance bounds, but experimental errors can compromise the quality of the resulting restraints, especially for weak cross peaks. In a new approach introduced here, termed RANDMARDI (random error MARDIGRAS), errors due to random noise and integration errors are mimicked by the addition of random numbers from within a specified range to each input intensity. Interproton distances are then calculated for the modified intensity set using MARDIGRAS. The distribution of distances that define the upper and lower distance bounds is obtained by using N randomly modified intensity sets. RANDMARDI has been used in the solution structure determination of the interstrand cross-link (XL) formed between 4-hydroxymethyl-4,5,8-trimethylpsoralen (HMT) and the DNA oligomer d(5-GCGTACGC-3)2 [Spielmann, H.P. et al. (1995) Biochemistry, 34, 12937–12953]. RANDMARDI generates accurate distance bounds from the experimental NOESY cross-peak intensities for the fixed (known) interproton distances in XL. This provides an independent internal check for the ability of RANDMARDI to accurately fit the experimental data. The XL structure determined using RANDMARDI-generated restrains is in good agreement with other biophysical data that indicate that there is no bend introduced into the DNA by the cross-link. In contrast, isolated spin-pair approximation calculations give distance restraints that, when applied in a restrained molecular dynamics protocol, produce a bent structure.Abbreviations NOE
nuclear Overhauser effect
- SD
standard deviation
- HMT
4-hydroxymethyl-4,5,8-trimethylpsoralen
- XL
psoralen-DNA interstrand cross-link 相似文献