IFNG and Mendelian susceptibility to mycobacterial diseases: Complete IFN-γ deficiency, IFNGR1 or IFNGR2 deficiency, and a number of other inborn errors of type II IFN immunity underlie Mendelian susceptibility to mycobacterial disease (MSMD) in humans, which is characterized by selective susceptibility to weakly virulent mycobacteria, such as the Bacille Calmette-Guerin (BCG) vaccine substrain and environmental mycobacteria, as well as to more virulent species of the Mycobacterium tuberculosis (Mtb) complex [170].