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   2887篇
  免费   194篇
  国内免费   7篇
  3088篇
  2024年   10篇
  2023年   24篇
  2022年   90篇
  2021年   129篇
  2020年   134篇
  2019年   246篇
  2018年   184篇
  2017年   137篇
  2016年   161篇
  2015年   148篇
  2014年   200篇
  2013年   268篇
  2012年   259篇
  2011年   241篇
  2010年   139篇
  2009年   105篇
  2008年   96篇
  2007年   126篇
  2006年   93篇
  2005年   71篇
  2004年   61篇
  2003年   53篇
  2002年   42篇
  2001年   8篇
  2000年   4篇
  1999年   7篇
  1998年   4篇
  1997年   8篇
  1996年   2篇
  1995年   4篇
  1994年   2篇
  1993年   1篇
  1992年   3篇
  1991年   2篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1986年   3篇
  1985年   1篇
  1984年   1篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1977年   2篇
  1976年   1篇
  1975年   4篇
  1974年   1篇
  1973年   1篇
  1970年   1篇
  1966年   2篇
排序方式: 共有3088条查询结果,搜索用时 15 毫秒
51.
The present study was designed to evaluate the effects of parenteral iron on red blood cell parameters, performance, and health in dairy Holstein calves. Twenty neonatal calves were equally divided at random into two groups, one of which served as controls. Care was taken to ensure homogeneity of sex, age, and general health status of the animals. The controls received a normal diet and water ad libitum, while the study animals were injected with 1 g iron as Fe-dextran 2 days after birth. A daily record was kept of the calves’ weight and growth parameters. At periods of 24–48 h after birth and at 14, 28, and 42 days of age, jugular blood was drawn from all the experimental and control animals to measure the packed cell volume, red blood count, hemoglobin, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, and iron levels. At the start of the experiments, there were no significant differences between these parameters between the two study groups (p > 0.05). With time, significant differences were seen between most of the values measured (p < 0.05) except for the mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, and iron level. Significant differences were seen for total weight gain and mean daily weight gain, which were higher in the iron-supplemented group (p < 0.05).  相似文献   
52.
CRC-associated P53 mutations have not been studied extensively in non-Western countries at relatively low CRC risk. We examined, for the first time, 196 paraffin-embedded CRC cases from Northern Iran for mutations in P53 exons 5-8 using PCR-direct sequencing. P53 status and mutation site/type were correlated with nuclear protein accumulation, clinicopathologic variables and data on K-ras mutations and high-level microsatellite instability (MSI-H). We detected 96 P53 mutations in 87 (44.4%) cases and protein accumulation in 84 cases (42.8%). P53 mutations correlated directly with stage and inversely with MSI-H. Distal CRCs were more frequently mutated at major CpG hotspot codons [248 (8/66, 12.1%), 175 (7/66, 10.6%), and 245 (7/66, 10.6%)], while in proximal tumors codon 213, emerged as most frequently mutated (5/28, 17.9% vs. 3/66, 4.5%, P = 0.048). Transitions at CpGs, the most common mutation type, were more frequent in non-mucinous (25% vs. 10.4% in mucinous, P = 0.032), and distal CRC (27% vs. 12.5% in proximal, P = 0.02), and correlated with K-ras transversions. Transitions at non-CpGs, second most common P53 mutation, were more frequent in proximal tumors (15.6% vs. 4.7% in distal, P = 0.01), and correlated with K-ras transitions and MSI-H. Overall frequency and types of mutations and correlations with P53 accumulation, stage and MSI-H were as reported for non-Iranian patients. However P53 mutation site/type and correlations between P53 and K-ras mutation types differed between proximal and distal CRC. The codon 213 P53 mutation that recurred in proximal CRC was previously reported as frequent in esophageal cancer from Northern Iran.  相似文献   
53.
The effects of Akacid plus, a novel guanidine-based polymer first introduced as a biocidal and disinfectant agent were studied on Aspergillus parasiticus growth and its aflatoxin (AF) productivity. The fungus was cultured on yeast extract-sucrose (YES) broth in presence of various twofold serial dilutions of 25% Akacid plus (1.5-96 microL/50 mL medium) and then incubated in shaking condition with 150 rev./min at 28 degrees C for 96 h. Based on obtained results, Akacid plus was found to significantly inhibit both growth and aflatoxin B1 (AFB1) synthesis in very low concentrations in a dose-dependent manner. Fungal growth inhibition was determined in the range of 9.6-99.6% in mycelia exposed to the total concentration range of 1.5-48 microL. A final concentration of 96 microL was necessary to completely inhibit the growth of fungus. Under similar conditions, AFB1 synthesis was found to be strongly inhibited by 8.1-98.0% in presence of 1.5-24 microL Akacid plus with a maximum of 100% by 48 microL concentration. With respect to the unique physico-chemical properties of Akacid plus, its marked inhibitory effects on A. parasiticus growth and its AFB1 synthesis shown for the first time in this study make it a promising candidate for application in prevention programmes of AF contamination of susceptible crops.  相似文献   
54.
The acyl carrier proteins (ACPs) of fatty acid synthase and polyketide synthase as well as peptidyl carrier proteins (PCPs) of nonribosomal peptide synthetases are modified by 4'-phosphopantetheinyl transferases from inactive apo-enzymes to their active holo forms by transferring the 4'-phosphopantetheinyl moiety of coenzyme A to a conserved serine residue of the carrier protein. 4'-Phosphopantetheinyl transferases have been classified into two types; the AcpS type accepts ACPs of fatty acid synthase and some ACPs of type II polyketide synthase as substrates, whereas the Sfp type exhibits an extraordinarily broad substrate specificity. Based on the previously published co-crystal structure of Bacillus subtilis AcpS and ACP that provided detailed information about the interacting residues of the two proteins, we designed a novel hybrid PCP by replacing the Bacillus brevis TycC3-PCP helix 2 with the corresponding helix of B. subtilis ACP that contains the interacting residues. This was performed for the PCP domain as a single protein as well as for the TycA-PCP domain within the nonribosomal peptide synthetase module TycA from B. brevis. Both resulting proteins, designated hybrid PCP (hPCP) and hybrid TycA (hTycA), were modified in vivo during heterologous expression in Escherichia coli (hPCP, 51%; hTycA, 75%) and in vitro with AcpS as well as Sfp to 100%. The designated hTycA module contains two other domains: an adenylation domain (activating phenylalanine to Phe-AMP and afterward transferring the Phe to the PCP domain) and an epimerization domain (converting the PCP-bound l-Phe to d-Phe). We show here that the modified PCP domain of hTycA communicates with the adenylation domain and that the co-factor of holo-hPCP is loaded with Phe. However, communication between the hybrid PCP and the epimerization domain seems to be disabled. Nevertheless, hTycA is recognized by the next proline-activating elongation module TycB1 in vitro, and the dipeptide is formed and released as diketopiperazine.  相似文献   
55.
Rajimehr R 《Neuron》2004,41(4):663-673
Recent findings have shown that certain attributes of visual stimuli, like orientation, are registered in cortical areas when the stimulus is unresolvable or perceptually invisible; however, there is no evidence to show that complex forms of orientation processing (e.g., modulatory effects of orientation on the processing of other features) could occur in the absence of awareness. To address these questions, different psychophysical paradigms were designed in six experiments to probe unconscious orientation processing. First we demonstrated orientation-selective adaptation and color-contingent orientation adaptation for peripheral unresolvable Gabor patches. The next experiments showed the modulatory effects of perceptually indiscriminable orientations on apparent motion processing and attentional mechanisms. Finally we investigated disappearance patterns of unresolvable Gabor stimuli during motion-induced blindness (MIB). Abrupt changes in local unresolvable orientations truncated MIB; however, orientation-based grouping failed to affect the MIB pattern when the orientations were unresolvable. Overall results revealed that unresolvable orientations substantially influence perception at multiple levels.  相似文献   
56.
Isolates of Cryptosporidium spp. from human and animal hosts in Iran were characterized on the basis of both the 18S rRNA gene and the Laxer locus. Three Cryptosporidium species, C. hominis, C. parvum, and C. meleagridis, were recognized, and zoonotically transmitted C. parvum was the predominant species found in humans.  相似文献   
57.
A comparative biochemical and structural study was performed on a cold active α-amylase from Bacillus cereus (BCA) and two well-known homologous mesophilic and thermophilic α-amylases from Bacillus amyloliquefaciens (BAA) and Bacillus licheniformis (BLA). In spite of a high degree of sequence and structural similarity, drastic variations were found for Topt as 50, 70 and 90 °C for BCA, BAA and BLA, respectively. The half-lives of thermoinactivation were 1 and 9 min for BCA and BAA at 80 °C respectively, whilst there was no inactivation for BLA at this temperature. Thermodynamic studies on inactivation process suggested that lower thermostability of BCA is due to lower inactivation slope of the Arrhenius plots and subsequently, lower Ea and ΔH#. Increased Km and accessible surface area for catalytic residues along with a decreased number of internal interactions in this region in BCA compared to BLA suggest that BCA substrate-binding site might be temperature sensitive and is probably more flexible. On the other hand, fewer ion pairs, destructive substitutions and disruption of aromatic interaction networks in structurally critical regions of Bacillus α-amylases result in a severe decrease in BCA thermostability compared to its mesophilic and thermophilic homologues.  相似文献   
58.
Breast cancer is a very heterogeneous disease, encompassing several intrinsic subtypes with various morphological and molecular features, natural history and response to therapy. Currently, molecular targeted therapies are available for estrogen receptor (ER) and human epidermal growth factor receptor 2 (Her2)-positive breast tumors. However, a significant proportion of primary breast cancers are negative for ER, progesterone receptor (PgR), and Her2, comprising the triple negative breast cancer (TNBC) group. Women with TNBC have a poor prognosis because of the aggressive nature of these tumors and current lack of suitable targeted therapies. As a consequence, the identification of novel relevant protein targets for this group of patients is of great importance. Using a systematic two dimensional (2D) gel-based proteomic profiling strategy, applied to the analysis of fresh TNBC tissue biopsies, in combination with a three-tier orthogonal technology (two dimensional PAGE/silver staining coupled with MS, two dimensional Western blotting, and immunohistochemistry) approach, we aimed to identify targetable protein markers that were present in a significant fraction of samples and that could define therapy-amenable sub-groups of TNBCs. We present here our results, including a large cumulative database of proteins based on the analysis of 78 TNBCs, and the identification and validation of one specific protein, Mage-A4, which was expressed in a significant fraction of TNBC and Her2-positive/ER negative lesions. The high level expression of Mage-A4 in the tumors studied allowed the detection of the protein in the tumor interstitial fluids as well as in sera. The existence of immunotherapeutics approaches specifically targeting this protein, or Mage-A protein family members, and the fact that we were able to detect its presence in serum suggest novel management options for TNBC and human epidermal growth factor receptor 2 positive/estrogen receptor negative patients bearing Mage-A4 positive tumors.Breast cancer, although a very heterogeneous disease, can be divided into three therapeutically relevant fundamental disease entities, simply based on estrogen receptor (ER) and human epidermal growth factor receptor 2 (Her2)1 status (i.e. ER+ and/or Her2+, and ERHer2), as the major currently available breast cancer therapeutic options are based on the ability to target these proteins. Hormone receptor positive and hormone receptor negative breast cancers are disease entities with distinct morphological, genetic and biological behavior (1). Hormone receptor negative tumors, which constitute ∼30% of primary breast cancers, tend to be high-grade, more frequently BRCA1 and TP53 mutated, and, more importantly, are not amenable to endocrine therapy. Her2 is amplified in ∼18–20% of breast cancers, and is more frequently observed in hormone receptor negative tumors. Her2 amplification is associated with worse prognosis (higher rate of recurrence and mortality) in patients with newly diagnosed breast cancer who do not receive any adjuvant systemic therapy. Her2 status is also predictive for several systemic therapies, particularly for agents that target Her2. The development of a humanized monoclonal antibody against Her2 (trastuzumab) has resulted in reduction of the risk of recurrence and mortality in patients with Her2 amplification (2, 3). Although trastuzumab is considered one of the most effective targeted therapies currently available in oncology, a significant number of patients with Her2-overexpressing breast cancer do not benefit from it (4, 5).Breast tumors that do not express ER, PgR, or Her2 (ER PgR Her2), as determined by immunohistochemistry (IHC), are generally referred to as triple negative breast cancers (TNBCs), and they are not candidates for targeted therapies (endocrine therapy or trastuzumab). Although TNBCs account for a relatively small proportion of breast cancer cases (10–15%), they are responsible for a disproportionate number of breast cancer deaths. TNBC tumors form a recognizable prognostic group of breast cancer with aggressive behavior that currently lacks the benefit of available systemic therapy (68). Given the need to develop molecular criteria to reproducibly categorize molecular breast tumor subtypes at the protein level and the lack of targeted therapies available to treat patients bearing TNBCs, we have implemented a systematic proteomics approach to identify, characterize, and evaluate proteins present in triple-negative tumors that could constitute an appropriate therapeutic target for the clinical management of this group of patients. To this end, based on the analysis of 78 individual TNBC samples, we have established a large, cumulative, 2D-PAGE database of proteins expressed by TNBCs, including some that could be of potential therapeutic value. Comparison of this TNBC protein database with protein databases of other breast cancer subtypes previously established by our laboratory allowed us to single out a number of proteins preferentially expressed in TNBCs for which targeted therapeutics exist. In this report we further focused on the characterization of one such target, the cancer/testis antigen, melanoma-associated antigen 4 - Mage-A4.Cancer/testis antigens (CTAs) are expressed in a large variety of tumor types, whereas their expression in normal tissues is restricted to male germ cells, which are immune-privileged because of their lack of or low expression of human leukocyte antigen (HLA) molecules (9). Several studies have shown the existence of natural cellular and humoral responses against some CTAs, indicating that they are appropriate targets for vaccine-based cancer immunotherapy (1012). So far, the use of CTAs in immunotherapeutic approaches to cancer treatment has been tested in more than 60 early phase clinical trials, with varying success, and a few candidate products have reached late-stage clinical trials. One such candidate vaccine, Astuprotimut-R (GSK-249553), a Mage-A3 antigen-specific cancer immunotherapeutic agent, is currently under clinical evaluation by GlaxoSmithKline in the largest-ever treatment trial in lung cancer, called MAGRIT (Mage-A3 as Adjuvant nonsmall cell lunG canceR ImmunoTherapy) (13).At present, CTAs comprise about 150 members, more than half of which are encoded by large, recently expanded families on chromosome X (14; see also CTDatabase at www.cta.lncc.br; last accessed 01.09.2012). These genes are organized into clusters and have undergone rapid evolution, possibly because of positive selection. The biological functions of CTAs are not fully understood, but emerging evidence suggest that they direct the proliferation, differentiation, and survival of human germ line cells and may have similar effect in cancer cells. Mage-A4 protein belongs to the Mage-A family of CT antigens. The Mage-A family is composed by 12 proteins (14, 15) and many members of the Mage-A family of CTAs have been associated with cancer, including breast cancer (14, 16, 17). However, past studies reported mostly on MAGE genes rather than protein expression, or on the expression of Mage protein families and not on any given specific protein.In this paper we describe the identification of Mage-A4 in breast tumor biopsies using 2D PAGE coupled with MS proteomics, and follow the protein localization from the tumor cells, to the tumor microenvironment, and to the serum of a patient. Using a three-tier orthogonal technology approach that combined 2D PAGE silver staining coupled with MS, with 2D Western blotting, and IHC, we showed that high level Mage-A4 expression in breast tumors occurs almost exclusively in the receptor negative disease (TNBC and Her2+ERPgR). The existence of immunotherapeutic approaches targeting MAGE protein family members (Mage-A4 specific or with broader specificity) and the fact that we were able to detect its presence in serum suggest novel management options for patients bearing Mage-A4 positive TNBCs and Her2+ERPgR tumors.  相似文献   
59.
60.
In vitro differentiation studies using the bipotential human leukemia cell line, HL60, have indicated that high levels of expression of two proto-oncogenes, c-fos and c-fms, are restricted to the myelomonocytic lineage. No such expression has been detected in induced granulocytic cells. In striking contrast to these observations, we found that c-fos mRNA levels are very high in purified human granulocytes, but barely detectable in blood monocytes and tissue macrophages. Human granulocytes contain, however, relatively low levels of c-fos protein, indicating that c-fos mRNA is inefficiently translated or that the protein is rapidly degraded in these cells. In closer agreement with the in vitro results, the level of the expression of c-fms is high in purified blood monocytes and undetectable in granulocytes. We found, however, that the evolution of monocytes into tissue macrophages is accompanied by a significant decrease in c-fms expression, suggesting that the function of c-fms is restricted to specific stages of monocytic differentiation. Our observations also show that results obtained using in vitro differentiation systems have to be regarded with caution, since they may not reflect the in vivo situation.  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