RB1BUS6lacgpt, a Marek's disease virus (MDV) mutant having a disrupted glycoprotein D (go) homolog gene, established infection and induced tumors in chickens exposed to it by inoculation or by contact. Lymphoblastoid cell lines derived from RB1BUS6lacgpt-induced tumors harbored only the mutant virus. These results provide strong evidence that an intact go homolog gene is not essential for oncogenicity or horizontal transmission of MDV.