Although prior studies from our group and others elucidated the individual effects of single-nucleotide polymorphisms (SNPs) in UBASH3A and PTPN22 on risk for T1D [9,16,26], the joint action of those SNPs in two independent T1D risk loci in disease development has not been well characterized. The gene discussed is PTPN22; the disease is type 1 diabetes mellitus.