The identification of four candidate genes directly involved in B cell receptor (BCR) signalling and B cell differentiation (PTPN22, BACH2, PTPN2, SH2B3), and three immunoregulatory cytokine genes (IL10, IL2/IL21, IL27) involved in the differentiation and function of specific B cell subsets, implicates a dysregulation in the B cell signalling pathway in the pathogenesis T1D. This evidence concerns the gene PTPN22 and type 1 diabetes mellitus.