STAT1 and immune system disorder: 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).