Germline gain-of-function (GOF) variants in the STAT3 (STAT3-GOF) gene cause early-onset immune dysregulation syndrome, which is characterized by lymphoproliferation and multi-organ autoimmunity, including type 1 diabetes mellitus, polyarthritis, inflammatory bowel disease (IBD), and interstitial pneumonia (IP) [3–7]. The gene discussed is STAT3; the disease is polyarticular arthritis.