 Most of these phamilies (~75%) do not have homologues outside of the mycobacteriophages and are of unknown function.Genetic studies with mycobacteriophage Giles show that 45% of the genes are nonessential for lytic growth.Host range analysis shows that not all mycobacteriophages from "M. smegmatis" infect other strains and only phages in Cluster K and in certain subclusters of Cluster A efficiently infect "M. tuberculosis" (Figure 1).