In addition to TCR signals, IL2 plays a key role, as indicated by defective thymic Treg development and subsequent autoimmunity in mice lacking IL2, or its high-affinity receptor chain CD25 (also called IL2Rα; see Box 6) (Furtado et al., 2002; Malek et al., 2002). Here, IL2 is linked to Autoimmunity.