First described in 2011, germline, monoallelic gain-of-function (GOF) mutations in the Signal Transducer and Activator of Transcription 1 (STAT1) gene result in a rare, combined immunodeficiency with a severe clinical phenotype which includes chronic mucocutaneous candidiasis (CMC), often in combination with various forms of autoimmunity as well as bacterial, viral or mycobacterial infections (1–3). The gene discussed is STAT1; the disease is Immunodeficiency.