The clinical course of this patient illustrates the classical yet variable manifestations of TYK2 deficiency, a form of Mendelian Susceptibility to Mycobacterial Disease (MSMD) characterized by vulnerability to intracellular pathogens, particularly weakly virulent mycobacteria such as Mycobacterium bovis BCG [1,6,7]. This evidence concerns the gene TYK2 and Mendelian susceptibility to mycobacterial diseases.