This is the reason that many characteristics of the two bacteria are similar. Insertion sequences of various types have been reported in mycobacteria. The first to be discovered was the one unique to M. paratuberculosis and named IS900. Genetic probes used for detection of M. paratuberculosis in clinical specimens or identification of M. paratuberculosis in cultures are based on detection of IS900. How these insertion elements affect the biology and pathogenic capacity of M. paratuberculosis or M. avium is not understood.
Evidence suggests, however, that they play a major role.