In the case example of the first young female patient given in Box 1, until the papers of Lilic et al. at the turn of the millennium and the seminal publication by van de Veerdonk et al. in 2011 [6–9], it was not understood that gain-of-function STAT1 mutations impairing IL-17 immunity lay behind the deregulation of cytokines and the defective Th1 and Th17 responses seen in the PID of chronic mucocutaneous candidiasis. The gene discussed is IL17A; the disease is chronic mucocutaneous candidiasis.