The activation of Treg cells in IgG4-related disease is suggested by the high expression of the forkhead box P3 (FOXP3) mRNA in tissue by the infiltrates of CD4+CD25+ Treg cells at affected sites and by the increased number of CD4+CD25+ Treg cells in the blood. The gene discussed is FOXP3; the disease is immunoglobulin G4-related sclerosing disease.