Abstract: A method is provided for attenuating a cytomegalovirus comprising functionally disrupting an open reading frame of a Toledo genome region or its homolog and making chimeric CMV virus genomes.

Abstract: The present invention relates to genetically engineered recombinant respiratory syncytial viruses and viral vectors which contain deletions of various viral accessory gene(s) either singly or in combination. In accordance with the present invention, the recombinant respiratory syncytial viral vectors and viruses are engineered to contain complete deletions of the M2-2, NS1, NS2, or SH viral accessory genes or various combinations thereof. In addition, the present invention relates to the attenuation of respiratory syncytial virus by mutagenisis of the M2-1 gene.