Besides MHC, the PTPN22 locus on 1p13 is well known to contribute to both T1D and RA [34],[35], and the WTCCC study reported three additional shared susceptibility loci (IL2RA on 10p15, PTPN2 on 18p11 and chromosome 12q14 region) [36]. The gene discussed is PTPN22; the disease is type 1 diabetes mellitus.