We shall then discuss the role of Treg cells in autoimmune diseases, especially how genetic anomalies such as FOXP3 gene mutations or genetic variations such as autoimmune disease-associated single nucleotide polymorphisms (SNPs) revealed by genome-wide association study (GWAS) affect Treg cell development and function, being causative of autoimmune diseases. The gene discussed is FOXP3; the disease is autoimmune disease.