Genetic characterization of mycobacteria has advanced considerably in the last decade. The identificatlon of putative virulence genes and their products has prompted investlgation of how expression of these factors is controlled during infection. Regulation of virulence determinants at the level of gene transcription has been investigated for many gram-negative pathogens and sheds considerable light on the control of gene expression during pathogenesis. However, extraction of intact mRNA from mycobacteria is difficult owing to the physical strength of the cell wall and has hindered the characterization of mycobacterial-gene expression