RESUMEN
The expressing cassette, LoxP-CMV-gpt-IRES-LoxP( about 2.9kb), was amplified by PCR from a plasmid, pIRES-gpt, by use of the primers , which contained the loxP sites in 5' terminals, respectively. The loxP sites were designed into primers by the software of Primer primer 5.0. Then the cassette was cloned into the site of BalI in pBUS10 to obtain pUS-gptIRES(L). The sequencing analysis for pUS-gptIRES(L) indicated that two loxP sites with the same direction were correctly inserted into pUS-gptIRES(L).The gpt gene in pUS-gptIRES(L) was replaced by a fragment including the full length GFP gene as well as SV40 poly A sequence to get pUS-GFPIRES(L). pUS-GFPIRES(L) was transiently transfected into CHO cell lines, and then the green fluorescence could be seen, the results showed that GFP gene could be expressed correctly. Moreover, pUS-GFPIRES(L) was transfected into the CEF infected MDV CVI988 strain and recombinant virus was selected by the green fluorescence. The growth curve of virus showed the characteristic of recombinant virus was the same as that of CVI988 in vitro. These results give the basis for further studying the characteristic of MDV in vivo and the application of the Cre/LoxP system to MDV genome.