There is substantial evidence that host genetic factors are important in determining susceptibility to mycobacteria. Several different techniques have been used to identify the genes involved. Studies of an inbred strain of mice with increased susceptibility to mycobacteria, salmonella and leishmania infections led to the identification of the natural resistance-associated macrophage protein gene (Nramp1). Case-control studies have confirmed the importance of the human equivalent of this gene, NRAMP1, and have also suggested that the major histocompatibility complex and vitamin-D receptor genes may be involved in determining human susceptibility to mycobacteria. Studies of individuals with the rare condition of increased susceptibility to disseminated bacille Calmette-Guerin and other atypical mycobacterial infections have identified several abnormalities in the genes encoding the interferon gamma receptor (IFNgammaR) ligand binding chain, IFNgammaR signal transduction chain, IFNgamma signal transduction and activation of transcription-1, interleukin 12 receptor beta1 subunit and interleukin 12 p40 subunit. A genome-wide linkage study has been performed to identify genes exerting a major effect on tuberculosis susceptibility in the general population. Linkages were found to markers on chromosomes 15 and X. Studies to identify the genes responsible are in progress.