The importance of PTPN2 in T cells in immune tolerance is highlighted by the development of autoimmunity in aged T‐cell‐specific PTPN2‐deficient mice on an otherwise non‐autoimmune C57BL/6 background (Wiede et al, 2011), the systemic inflammation and autoimmunity evident when PTPN2 is deleted in the hematopoietic compartment of adult C57BL/6 mice (Wiede et al, 2017b) and the accelerated onset of type 1 diabetes in T‐cell‐specific PTPN2‐deficient mice on the autoimmune‐prone non‐obese diabetic (NOD) background (Wiede et al, 2019). The gene discussed is PTPN2; the disease is type 1 diabetes mellitus.