Transdifferentiation of Th17 cells is well documented (21–23, 26, 53) and a late developmental switch to IFN-γ expression in Th17 cells has been implicated in the pathologies of a diverse group of inflammatory autoimmune diseases such as psoriatic arthritis, Crohn’s disease, ulcerative colitis, type 1 diabetes, and multiple sclerosis (23, 26, 49–52, 54). This evidence concerns the gene IFNG and psoriatic arthritis.