Indeed, the missense Arg → Trp (R620W) polymorphism of the eCB synthesizing enzyme PTPN22 [encoding lymphoid protein tyrosine phosphatase (LYP), which is important in negatively controlling activation of T lymphocytes] was found to be associated with increased risk of type 1 diabetes mellitus (T1DM), rheumatoid arthritis (RA), juvenile idiopathic arthritis, systemic lupus erythematosus, Graves disease, myasthenia gravis, generalized vitiligo, and granulomatosis with polyangiitis [previously known as Wegener’s granulomatosis; reviewed in Ref. Here, PTPN22 is linked to diabetes mellitus.