T1D endotype-1 includes patients diagnosed at young age (< 7-years old) and characterized with early beta cell destruction and aggressive insulitis with abundant infiltration of CD8+ T cells and CD20+ B cells, whereas T1D endotype-2 includes patients diagnosed in adolescence or adulthood that have fewer infiltrating CD20+ B cells (13, 14). The gene discussed is CD8A; the disease is type 1 diabetes mellitus.