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   10069篇
  免费   815篇
  国内免费   2篇
  2023年   55篇
  2022年   114篇
  2021年   229篇
  2020年   148篇
  2019年   187篇
  2018年   239篇
  2017年   200篇
  2016年   341篇
  2015年   541篇
  2014年   613篇
  2013年   712篇
  2012年   872篇
  2011年   887篇
  2010年   576篇
  2009年   454篇
  2008年   594篇
  2007年   599篇
  2006年   516篇
  2005年   533篇
  2004年   451篇
  2003年   443篇
  2002年   369篇
  2001年   116篇
  2000年   89篇
  1999年   113篇
  1998年   104篇
  1997年   68篇
  1996年   62篇
  1995年   62篇
  1994年   51篇
  1993年   52篇
  1992年   38篇
  1991年   36篇
  1990年   41篇
  1989年   28篇
  1988年   22篇
  1987年   20篇
  1986年   25篇
  1985年   18篇
  1984年   23篇
  1983年   26篇
  1982年   26篇
  1981年   20篇
  1980年   11篇
  1979年   12篇
  1978年   28篇
  1977年   15篇
  1973年   10篇
  1972年   15篇
  1968年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Despite the combined use of surgery and chemoradiotherapy, the poor prognosis of advanced non-smallcell lung cancer (NSCLC) requires the definition of new therapeutic approaches. The presence of T lymphocytes, with peculiar phenotypic, functional and molecular characteristics within the tumour, suggested the possible use of these cells, expanded in vitro, in protocols of adoptive immunotherapy. We have described how a population of oligoclonal T lymphocytes, derived from advanced NSCLC, can be expanded in vitro and has the capability of lysing autologous cancer cells. What is more important, we observed that patients with advanced NSCLC, treated with TIL expanded in vitro and recombinant interleukin-2, seemed to have a disease-free period longer than that of patients treated with conventional chemoradiotherapy. in an attempt to find new sources of specific lymphocytes for immunotherapy, we describe the analysis of the phenotypic, functional and molecular characteristics of T lymphocytes, derived from lymph nodes draining advanced NSCLC. In this paper we show that these cells, have restriction patterns of T cell receptor chain similar to those detectable in the population of infiltrating T lymphocytes. This finding suggests that T cells derived from draining lymph nodes of advanced NSCLC have peculiar characteristics and can be a suitable source of effector cells for protocols of adoptive immunotherapy in lung cancer treatment.  相似文献   
92.
On the statistical assessment of similarities in DNA sequences   总被引:3,自引:2,他引:1       下载免费PDF全文
The statistical behavior of the similarity score for unrelated DNA sequences calculated as letter-by-letter comparison or from various forms of optimal alignment was studied. It was found that natural DNA-sequences from a data base and true random sequences show the same statistical behavior in terms of such scores. This makes it possible to adopt a simple criterion for the rejection of fortuitous similarity. It is based on the mean and standard deviation of chance scores whose expected values, depending on chain length, gap penalty and probability of letter coincidence, may be calculated from formulae given in the paper.  相似文献   
93.
Abstract A circular restriction map of the genome of the phage L ( Salmonella typhimurium ) has been constructed with five restriction endonucleases, Eca I, Eco RI, Bam HI, Bgl I, and Pst I. The Eco RI fragments of phage-L DNA were cloned into pACYC184, and the resulting recombinant plasmids pL1, pL2,…,pL7 were introduced into Salmonella typhimurium . The genes present on the fragments cloned were identified by the marker rescue experiments with the L-phage amber mutants. A physical gene map of the L genome obtained in this way was compared with that of P22.  相似文献   
94.
Summary We have studied the frequency of trisomics in newly formed zygotes and the proportion of trisomics, k, coming from consanguineous marriages by assuming that recessive genes at a single locus or multiple loci are responsible for the induction of nondisjunction. For mitotic nondisjunction, the value of k increases as the magnitude of consanguinity of the parents increases, but the opposite relationship holds for meiotic nondisjunction. Therefore, it is important to distinguish mitotic and meiotic types in the genetic study of nondisjunction. This seems to be one of the simples tests for detecting the genetic control of nondisjunction.  相似文献   
95.
Summary A new cell line, SS78, was established from a primary renal cell carcinoma of a Caucasian male. The tissue was dispersed with collagenase, and viable cells were separated by flotation on a Ficoll-Hypaque gradient. In culture, the SS78 cells retained a distinct epithelial morphology, and no fibroblastlike cells were seen. The cultured cells were aneuploid with a modal chromosome number of 80 and had several marker chromosomes. Inoculation of the cultured cells into athymic nude mice caused tumors at the sites of inoculation. This research was supported in part by Grants CA 15972 and CA 14930 from the National Cancer Institute through the National Bladder Cancer Project and by the Medical Research Service of the Veterans Administration.  相似文献   
96.
The dye eriochromblack T (erio T), added to an aqueous suspension of bovine retinal outer segments solubilized by digitonin, shows a light-induced absorption-increase at =645 nm. Erio T is shown to directly interact with micellar metarhodopsm I and metarhodopsin II. The absorption-changes of erio T can be regarded as an indication of the transition from the metarhodopsin I conformation (with associated Ca2+) to the metarhodopsin II conformation (with associated H+).Thanks are due to the Deutsche Forschungsgemeinschaft for financial support (Em 18/1-4).  相似文献   
97.
The etioplast»chloroplast transition in the cotyledons of mustard seedlings (Sinapis alba L.) has been studied by electron microscopy. It was found that the active form of phytochrome, established by a red light pulse pretreatment, increases the initial rate and eliminates the lag of grana and stroma thylakoid formation after the onset of white light 60 h after sowing. The effect of a pretreatment with 15 s red light pulses is fully reversible by 756 nm light pulses. This reversibility is lost within 5 min. Evidence is presented which suggests that the time course of grana and stroma thylakoid formation is not correlated with the time course of the dispersal of the prolamellar body. The different functions of phytochrome and chlorophyll in controlling thylakoid formation are discussed.  相似文献   
98.
Recent studies have shown marked increases in brain content of neuropeptide Y (NPY) after seizures induced by intraperitoneal injection of kainic acid and after pentylenetetrazole kindling in the rat. We have now investigated possible changes in the rate of biosynthesis of NPY after kainic acid treatment, by using pulse-labeling of the peptide and by determining prepro-NPY mRNA concentrations. For pulse labeling experiments, [3H]tyrosine was injected into the frontal cortex, and the incorporation of the amino acid into NPY was determined after purifying the peptide by gel filtration chromatography, antibody affinity chromatography, and reversed-phase HPLC. At 2 and 30 days after kainic acid treatment, the rate of tyrosine incorporation was enhanced by approximately 380% in the cortex. In addition, concentrations of pre-pro-NPY mRNA were determined in four different brain areas by hybridization of Northern blots with a complementary 32P-labeled RNA probe 2, 10, 30, and 60 days after kainic acid treatment. Marked increases were observed in the frontal cortex (by up to 350% of controls), in the dorsal hippocampus (by 750%), and in the amygdala/pyriform cortex (by 280%) at all intervals investigated. In the striatum only a small, transient increase was observed. The data demonstrate increased expression of prepro-NPY mRNA and an enhanced rate of in vivo synthesis of NPY as a result of seizures induced by the neurotoxin kainic acid.  相似文献   
99.
100.
The sequence selectivity of enzyme-DNA interactions was analyzed by comparing discrimination between synthetic oligonucleotides containing the canonical site GAATTC and altered DNA sequences with the EcoRI DNA methyltransferase. The specificities (kcat/KmDNA) are decreased from 5- to 23,000-fold relative to the unmodified site. For several substrates the decrease in kcat makes a disproportionate contribution to the specificity difference, suggesting that discrimination is mediated by the placement of critical catalytic residues rather than binding interactions. This is supported by our observation that specificity changes are generally not followed by changes in the stability of the methyltransferase-DNA complexes. Also, base pair substitutions near the site of methylation result in greater decreases in complex stability, suggesting that recognition and catalytic mechanisms overlap.  相似文献   
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