Among the T1D genomic risk loci identified by genome-wide association studies, at least six harbour candidate genes that have been implicated directly in B cell differentiation and function (PTPN22, BACH2, PTPN2, IL2–IL21, IL10 and SH2B3; http://www.T1DBase.org). The gene discussed is PTPN22; the disease is type 1 diabetes mellitus.