Transdominant repressors of viral gene phenotypic expression derived from the rev gene product of HIV-1 or the rex gene product of HTLV-1 and corresponding mutated genes, having the capability of repressing the Rev function in HIV-1 and/or the Rex function in HTLV-I and HTLV-II and, in some cases, both the Rev and the Rex function and are, therefore, active in more than one viral species. Such transdominant viral mutants are useful as anti-viral agents to, for example protect cells against the deleterious effects of viral, e.g. HIV-1, infection.