The concept of genetic suppression of phenotypes has existed since the original description of suppression of eye color in Drosophila (Sturdevant 1920). Subsequently, genetic suppression was recognized and characterized in a variety of prokaryotic and eukaryotic organisms (Gorini and Beckwith 1966; Hartman and Roth 1973) and was demonstrated to occur through a variety of mechanisms (Gorini 1970; Hartman and Roth 1973). Of particular use for genetic analysis of prokaryotic viruses were host-dependent suppressor-sensitive mutations, which were conditionally lethal depending on the host bacterial strain used as an indicator for viral growth (Edgar 1966). The first documentation of genetic suppression in an animal virus was in reovirus (Ramig et al. 1977). Suppression was subsequently documented in a number of animal viruses, including papovavirus (Shortle et al. 1979), picornavirus (King et al. 1980), vaccinia virus (McFadden et al. 1980), influenza virus (Tolpin et al. 1981), herpesvirus (Hall and Almey 1982), and adenovirus (Kruijer et al. 1983), demonstrating the general applicability of the concept to animal virus genetics.