全文获取类型
收费全文 | 12721篇 |
免费 | 1067篇 |
国内免费 | 810篇 |
出版年
2024年 | 11篇 |
2023年 | 195篇 |
2022年 | 357篇 |
2021年 | 675篇 |
2020年 | 416篇 |
2019年 | 551篇 |
2018年 | 571篇 |
2017年 | 382篇 |
2016年 | 562篇 |
2015年 | 775篇 |
2014年 | 868篇 |
2013年 | 990篇 |
2012年 | 1170篇 |
2011年 | 1022篇 |
2010年 | 643篇 |
2009年 | 557篇 |
2008年 | 606篇 |
2007年 | 574篇 |
2006年 | 505篇 |
2005年 | 421篇 |
2004年 | 350篇 |
2003年 | 275篇 |
2002年 | 241篇 |
2001年 | 223篇 |
2000年 | 196篇 |
1999年 | 202篇 |
1998年 | 114篇 |
1997年 | 139篇 |
1996年 | 117篇 |
1995年 | 97篇 |
1994年 | 104篇 |
1993年 | 59篇 |
1992年 | 97篇 |
1991年 | 77篇 |
1990年 | 66篇 |
1989年 | 74篇 |
1988年 | 66篇 |
1987年 | 43篇 |
1986年 | 34篇 |
1985年 | 34篇 |
1984年 | 29篇 |
1983年 | 20篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
Sister-chromatid exchange (SCE) induction by the direct-acting bifunctional carcinogen, diepoxybutane (DEB), was investigated in multiple tissues in vivo. The log-log dose SCE response relationship was found to be parallel to that previously reported for DEB induction of lung adenomas. However, the SCE assay is approximately 20 times as sensitive in detecting genotoxic effects of DEB than indicated by the lung adenoma assay. Examination of second and third division cells following various treatment protocols revealed that regardless of the nature of initially induced lesions, they are rapidly repaired with no evidence of persistence beyond 1 cell cycle. 相似文献
72.
The exchange behavior of the guanine N(1) and uracil N(3) protons in the self-complementary hexanucleotide r(ApApGpCpUpU) has been studied at 5 degrees C in 80% H2O/20% D2O by proton NMR. Under these conditions, the hexanucleotide forms a stable miniduplex. The exchange rate of all Watson-Crick NH protons is unaffected by addition of trifluoroethylamine up to 0.07 M. On the other hand, addition of phosphate buffer, pH 6.9, enhances the exchange rate of the uracil N(3) protons of both terminal and internal A X U base pairs but does not influence the exchange rate of the guanine N(1) protons of the central G X C base pairs. Catalysis by increased phosphate concentrations results in an open-limited rate of the internal A X U base pairs with kex = 233 s-1, equivalent to a lifetime of 4.3 ms. The proton exchange of the central G X C is regulated by the opening rate of the central core of the miniduplex. On the other hand, the sensitivity of the exchange rate of internal as well as of terminal A X U base pairs can be explained by their reduced lifetime due to end "fraying" and a subsequent catalysis of the exchange process from the opened state. These results suggest that it may be possible to probe labilized parts of RNAs such as tRNA by gradual addition of the exchange catalyst phosphate and to monitor their exchange rates by proton NMR. 相似文献
73.
Jila H. Boal Scott F. Deamond Daniel E. Callahan Sarah A. Bruce Paul O. P. Ts'o L. L. Kan 《Cell biochemistry and biophysics》1989,14(3):245-256
The nuclear magnetic resonance (NMR) parameters, spin-lattice (T1), and spin-spin (T2) relaxation time, are usually longer for neoplastic cells than for normal cells of the same cell type. This has generally
been true at low NMR frequencies (≤100 MHz) when comparisons have been made between normal and neoplastic cells that have
both spent a short time in culture. We have previously demonstrated that although the T1 values of paired normal and neoplastic Syrian hamster (SH) fibroblastic cells in culture are not significantly different
when measured at 300 MHz, the 300 MHz T2 values for the neoplastic cells are smaller than those of the normal cells. (Xin et al. (1986),Cell Biophysics
8, 213.) Since treatment of normal diploid cells with polypeptide growth factors or tumor promoters frequently results in reversible
expression of neoplasia-associated phenotypes, T1 and T2 were obtained at 300 MHz for treated and untreated SH cells to see if these compounds could also produce smaller 300 MHz
T2 values. Secondary culture SH fetal fibroblast cells were treated with epidermal growth factor (EGF), fibroblast growth factor
(FGF), phorbol-12,13-didecanoate (PDD) and 4-α-phorbol-12,13-didecanoate (4αPDD). Treatment with either growth factor resulted
in smaller T2 values, but a statistically significant decrease was not observed for PDD or 4αPDD. The observed reductions in T2 values were correlated with the morphological and growth-stimulatory effects of these compounds on the cells. 相似文献
74.
75.
76.
77.
S. K. Hira J. Kamanga G. J. Bhat C. Mwale G. Tembo N. Luo P. L. Perine 《BMJ (Clinical research ed.)》1989,299(6710):1250-1252
OBJECTIVE--To determine the occurrence of vertical transmission of HIV-I from women positive for the virus and the prognosis for their babies. DESIGN--Women presenting in labour were tested for HIV-I. Their newborn babies were also tested. Women positive for the virus were followed up with their babies for two years. SETTING--Teaching hospital in Lusaka, Zambia. SUBJECTS--1954 Women, of whom 227 were seropositive. Of 205 babies, 192 were positive for HIV-I. After birth 109 seropositive mothers and their babies and 40 seronegative mothers and their babies were available for follow up. MAIN OUTCOME MEASURES--Serological examination of mothers and their babies by western blotting. Birth weight and subsequent survival of babies. Women and babies were tested over two years for signs of seroconversion and symptoms of infection with HIV, AIDS related complex, and AIDS. RESULTS--Of the 109 babies born to seropositive mothers and available for follow up, 18 died before 8 months, 14 with clinical AIDS. Of the 91 remaining, 23 were seropositive at 8 months. By 24 months 23 of 86 surviving babies were seropositive, and a further five infected babies had died, four were terminally ill, 17 had AIDS related complex, and two had no symptoms. The overall rate of perinatal transmission was 42 out of 109 (39%). The overall mortality of infected children at 2 years was 19 out of 42 (44%). Before the age of 1 year infected children had pneumonia and recurrent coughs, thereafter symptoms included failure to thrive, recurrent diarrhoea and fever, pneumonia, candidiasis, and lymphodenopathy. All babies had received live attenuated vaccines before 8 months with no adverse affects. CONCLUSIONS--Vertical transmission from infected mothers to their babies is high in Zambia and prognosis is poor for the babies. Perinatal transmission and paediatric AIDS must be reduced, possibly by screening young women and counselling those positive for HIV-I against future pregnancy. 相似文献
78.
Virological survey of rhesus monkeys in China 总被引:1,自引:0,他引:1
A virological survey of rhesus monkeys captured in China for 13 viruses and/or antibodies was performed. Antigens used were SFV, SF40, HSV-1, Sa11, measles, vaccinia, epidemic or simian hemorrhagic fever, Langat, Kunming, poliomyelitis, HIV, SV41 and rubella. Monkeys were from Sichuan, Hunan, Guizhou, Yunnan and Guangxi provinces. Antibody was detected to all the listed viruses except HIV, SV41 and rubella. Both SFV and SV40 were recovered from monkeys, but H. simiae, LCM and coxsackieviruses were not. 相似文献
79.
Expression and function of the UM4D4 antigen in human thymus 总被引:3,自引:0,他引:3
D A Fox L S Chan L Kan O Baadsgaard K D Cooper 《Journal of immunology (Baltimore, Md. : 1950)》1989,143(7):2166-2175
UM4D4 is a newly identified T cell surface molecule, distinct from the Ag receptor and CD2, which is expressed on 25% of peripheral blood T cells, resting or activated. Monoclonal anti-UM4D4 is mitogenic for T cells and T cell clones. Since alternative activation pathways independent of Ag/MHC recognition may be important in thymic differentiation, the expression and function of UM4D4 was examined in human thymus. UM4D4 was found on the surface of 6% of thymocytes. All thymocyte subsets contained UM4D4+ cells but expression was greatest on thymocytes that were CD1- (12%), CD3+ (11%) and especially CD4-CD8- (18%). CD3+CD4- CD8- cells, most of which bear the gamma delta-receptor, were greater than or equal to 50% + for UM4D4. Moreover, anti-UM4D4 was comitogenic for thymocytes together with PMA or IL-2. Anti-UM4D4 also reacted strongly with a subset of thymic epithelial cells in both cortex and medulla. Dual color fluorescence microscopy, with anti-UM4D4 and antibodies to other thymic epithelial Ag, showed UM4D4 expression on neuroendocrine thymic epithelium but not on thymic fibrous stroma. Thus, UM4D4 is expressed on, and represents an activation pathway for, a subset of thymic T cells. In addition, this determinant, initially identified as a novel T cell activating molecule, is broadly expressed by neuroendocrine thymic epithelium. Although the function of UM4D4 on the thymic epithelial cells is not yet clear, it is possible that UM4D4 represents a pathway for the functional activation of a subset of the thymic epithelium as well as a subset of thymocytes, thus playing a dual role in T cell differentiation. 相似文献
80.
Norimichi Kan Takashi Okino Masaki Nakanishi Kohei Satoh Kazuhisa Ohgaki Takayoshi Tobe 《Cancer immunology, immunotherapy : CII》1989,28(4):260-266
Summary The synergistic antitumor effect of interleukin-2(IL-2)-cultured tumor-bearer spleen cells (cultured lymphocytes) and immune fresh spleen cells was examined. Tumor-bearer cultured lymphocytes were obtained by culturing BALB/c spleen cells from syngeneic MOPC104E-tumor-bearing mice for 11 days with crude IL-2 and a soluble tumor extract. These cultured lymphocytes had weak antitumor activity when transferred i.p. into tumor-bearing mice that had been inoculated i.p. with 105 tumor cells 5 days previously. Immune fresh spleen cells, obtained from mice in complete remission after the treatment with cyclophosphamide, also had weak antitumor activity when transferred at the same schedule. The cultured cells and the fresh cells, mixed together before transfer, significantly augmented the therapeutic effect. At least 1×107 tumor-bearer cultured lymphocytes and 4×107 immune cells were needed for the synergistic effect. A tumor-specific combination was needed for both cultured and fresh cells. The effective subpopulation of tumor-bearer cultured lymphocytes was a cytotoxic one from an Lyt2+ precursor, and that of the immune fresh spleen cells was noncytotoxic, Lytl+ and Lyt2+ T-cells.A similar synergistic effect was also observed during in vitro coculture of tumor-bearer and immune cells. Cytotoxicity, as assessed by the 51Cr-release test, of tumor-bearer IL-2-cultured lymphocytes was maintained most effectively after 3 or 4 days of culture without IL-2 when the lymphocytes were cocultured with immune fresh spleen cells and tumor cells. 相似文献