This partial IFN-γ deficiency is milder than the profound deficits seen in genetic etiologies of MSMD, such as complete T-bet and RORγT deficiencies (43, 45), probably accounting for the absence of vulnerability to weakly virulent mycobacteria in LY9-deficient individuals. The gene discussed is IFNG; the disease is Mendelian susceptibility to mycobacterial diseases.