IFNG and Mendelian susceptibility to mycobacterial diseases: The study of Mendelian susceptibility to mycobacterial disease (MSMD), a rare condition that selectively predisposes ∼1/50,000 individuals to severe disease caused by weakly virulent mycobacteria (i.e., Bacille Calmette-Guérin [BCG] vaccine substrain and environmental mycobacteria), has identified IFN-γ as a key component of human antimycobacterial immunity (Bustamante, 2020).