Selective susceptibility to poorly pathogenic mycobacteria, such as bacille Calmette-Guérin (BCG) vaccine and environmental non-tuberculous mycobacteria (NTM), has long been suspected to be a mendelian disorder but its molecular basis has remained elusive. Recently, recessive mutations in the interferon gamma receptor ligand-binding chain (IFNgR1), interferon gamma receptor signalling chain (IFNgR2), interleukin 12 p40 subunit (IL-12p40), and interleukin 12 receptor beta 1 chain (IL-12Rb1) genes have been identified in a number of patients with disseminated BCG or NTM infection. Although genetically distinct, these conditions are immunologically related and highlight the essential role of interferon gamma-mediated immunity in the control of mycobacteria in man.