Genetic manipulation of mycobacteria has historically been difficult. This is in large part due to the impenetrable nature of the cell wall, resulting in difficulty both in introducing DNA into the bacterium and subsequent isolation of intact plasmid DNA. In addition, the mycobacterial cell wall contains complex lipids and polysaccharides that can contaminate DNA preparations. The hydrophobic nature of the cell wall results in cells clumping in culture, hampering the isolation of clonal populations important for many molecular biological purposes. In spite of these obstacles, the advent of efficient mycobacterial transformation systems (1) resulted in an explosion of research into plasmid vectors and numerous genetic systems for Mycobacterium tuberculosis have now been described.