Late-stage developmental plasticity is potentially perilous: interferon-γ (IFN-γ+) Th17 cells have been implicated in several human autoimmune diseases including inflammatory bowel disease (Annunziato et al., 2007), juvenile idiopathic arthritis (Nistala et al., 2010), and multiple sclerosis (Kebir et al., 2009); ex-Foxp3+ Th17 cells play a pathogenic role in rheumatoid arthritis (Komatsu et al., 2014); and interleukin-17 (IL-17+) Th2 cells have been positively linked to the severity of asthma (Irvin et al., 2014). Here, IL17A is linked to multiple sclerosis.