Technical Abstract: Gallid herpesvirus 2 (GaHV-2), commonly known as Marek’s disease virus serotype-1 (MDV-1), causes T cell lymphoma in chickens. Vaccines prepared from the CVI988/Rispens MDV-1 strain currently offers the best protection. Although, CVI988 is non-oncogenic, it codes for at least two forms of the MDV oncoprotein Meq, and these proteins (CVI-Meq and CVI-L Meq) have not previously been characterized. Here, we report that both CVI-Meq proteins, like the Meq protein of Md5 (a very virulent oncogenic strain) were capable of transforming Rat-2 and NIH3T3 cells. CVI-Meq proteins activated the meq promoter only in the presence of chicken c-Jun (CK-Jun) whereas Md5-Meq activated the same promoter irrespective of CK-Jun co-expression. However, Meq proteins of both Md5 and CVI988 bound the meq promoter in a ChIP assay regardless of whether CK-Jun was co-expressed or not. Domain swapping experiment indicated that both DNA binding and transactivation domains of Md5 Meq are essential for its full transactivation function. Further, site-directed mutagenesis of the Md5-Meq protein showed that amino acid residues at positions 71 and 320 are necessary for increasing transcription from its own promoter. All three Meq proteins activated the MDV gB, MMP-3 and Bcl-2 promoters and suppressed transcription from the MDV pp38/pp14 bidirectional promoter. Collectively, our data suggest that CVI-Meq proteins are generally weak transactivators, what might contribute to non-oncogenic phenotype of CVI988 virus in chickens.