It has been demonstrated previously that the autoimmunity-associated tyrosine phosphatase PTPN22 restrains T cell responses to weak affinity and/or self-antigens,5 and that systemic inhibition or deletion of PTPN22 improves responses to tumors.6 7 PTPN22 deletion restricted to T cells is sufficient to induce enhanced tumor control, as demonstrated by tumor rejection after adoptive transfer of Ptpn22KO naïve, effector or memory T cells into tumor bearing wild type host mice.8 9. This evidence concerns the gene PTPN22 and neoplasm.