The Marek's disease virus (MDV) induces T-cell tumors in susceptible chickens. Of the 80 to 100 known MDV genes, only the MDV MEQ gene was shown to have transforming properties. Further evidence that MEQ is probably the principal oncogene in MDV came when researchers used overlapping cosmid clones of MDV and demonstrated that deleting MEQ resulted in a highly protective Marek's disease (MD) vaccine. We deleted both copies of MEQ from a bacterial artificial chromosome clone (BAC) of MDV. The virus, BACdelMEQ, was completely attenuated and did not appear to have any adverse effect on chicken body weight in MDV maternal-antibody-positive chickens, as measured at 8 wk of age. In two protection studies, BACdelMEQ efficiently protected susceptible chickens from a challenge by MDV strain 686, one of the most virulent MDV strains. In both protection studies, the BACdelMEQ protected chickens significantly better than the commercial MD vaccine, CVI988/Rispens. Only the protein-coding sequences of MEQ were deleted and all upstream and downstream regulatory sequences were left intact. Thus, BACdelMEQ has the potential to be a superior MD vaccine as well as a vector to deliver various foreign genes to poultry.