These disorders encompass defects of IFN-γ production or response to IFN-γ, caused by mutations in IL12B, IL12RB1, ISG15, NEMO, IFNGR1, IFNGR2, STAT1, NEMO, IRF8, and CYBB. Altogether, they constitute 18 genetic etiologies of MSMD based on the mode of inheritance, complete or partial defect, expression of the mutant allele, and the functional aberrations. The gene discussed is IFNG; the disease is Mendelian susceptibility to mycobacterial diseases.