Thousands of mycobacteriophage have been isolated using a single host strain, "Mycobacterium smegmatis" mc2155, over 1400 of which have been completely sequenced. These are mostly from environmental samples, but mycobacteriophages have also been isolated from stool samples of tuberculosis patients, although these have yet to be sequenced. About 30 distinct types (called clusters, or singletons if they have no relatives) that share little nucleotide sequence similarity have been identified. Many of the clusters span sufficient diversity that the genomes warrant division into subclusters (Figure 1).