Fig. 2: Representation of the process whereby antigen (in this case peptides of proinsulin) is presented to CD4 T cells by human leukocyte antigen (HLA) class II molecules on the antigen presenting cell. This results in T-cell activation. In this diagram the 4 major genes associated with type 1 diabetes are present. CTLA-4 is an inhibitor of T-cell activation, as is lymphoid tyrosine phosphatase (LYP), which is encoded by the gene PTPN22. The complex of LYP–C-terminal Src kinase (CSK) inhibits Lck signalling after engagement of the T-cell antigen receptor (TCR). Photo by: Lianne Friesen and Nicholas Woolridge