Inducible T cell costimulator (ICOS) signaling is important in thymic development of IL-17A–producing γδ T cells and function during experimental autoimmune encephalomyelitis (Buus et al., 2016; Galicia et al., 2009), while the function of PD-1 on these cells is unknown. The gene discussed is ICOS; the disease is experimental autoimmune encephalomyelitis.