The invention relates to a family of DNA insertion sequences (ISM) of mycobacterial origin and other DNA probes which may be used as probes in assay methods for the identification of mycobacteria and the differentiation between closely related mycobacterial strains and species. In one method the probes are used to distinguish pathogenic M. paratuberculosis from M. avium, which finds an application in the diagnosis of Crohn's disease in humans and Johne's disease in animals. The use of ISM, and of proteins and peptides encoded by ISM, in vaccines, pharmaceutical preparations and diagnostic test kits is also disclosed.