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Tobacco plants were transformed with the movement protein (pathogenicity) gene (BC1) from tomato mottle gem-inivirus (TMoV), using Agrobacterium-mediated transformation. Different transgenic tobacco lines that expressed high levels of the BC1 protein had phenotypes ranging from plants with severe stunting and leaf mottling (resembling geminivirus symptoms) to plants with no visible symptoms. The sequence data for the BC1 transgene from the transgenic plants with the different phenotypes indicated an association of spontaneously mutated forms of the BC1 gene in the transformed tobacco with phenotype variations. One mutated transgene associated with an asymptomatic phenotype had a major deletion at the C terminus of 119 amino acid residues with a recombination resulting in the addition of 26 amino acid residues of unidentified origin. This asymptomatic, mutated BC1 attenuated the phenotypic expression of the symptomatic BC1 in a tobacco line containing both copies of the BC1 gene. Another mutated form of the BC1 gene amplified from an asymptomatic, multicopy transgenic tobacco plant did not induce symptoms when transiently expressed in tobacco via a virus vector. The symptom attenuation in the transgenic tobacco by the asymptomatic BC1 may involve trans-dominant negative interference.