GX0101 was the first reported field strain of recombinant Mareks disease virus (MDV) that contained a long terminal repeat (LTR) from the reticuloendotheliosis virus (REV).It is a very virulent MDV strain,with relative high horizontal transmission ability.The REV LTR in GX010I genome was proved to decrease the pathogenicity but increase the potential for horizontal transmission of the virus.This study constructed recombinant MDV GXOI01-ALV-LTR to study stability of avian leukosis virus (ALV) LTR at the REV LTR insertion site in GXOI01 genome and its influence on biological activities of the recombinant virus.The results showed that GX010I-ALV-LTR was able to replicate stably both in vitro and in vivo.ALV LTR remained stable in chickens infected either by inoculation with the recombinant virus GXOI01-ALV-LTR or by horizontal transmission,as well as in cell culture.The pathogenic properties of GXOIOI-ALV-LTR virus were evaluated in infected SPF chickens.The study demonstrated that the GXOIOI-ALV-LTR virus had a weaker inhibitory effect on the growth rates of the infected chickens and induced weaker immunosuppressive effects.Horizontal transmission ability of the GXOIOI-ALV-LIR virus appeared to be similar with its parental virus GX010I.In short,ALV LTR was stable in GXOI01 after replacing REV LTR,and the recombinant virus showed similar horizontal transmission ability but decreased pathogenicity.