Whereas there are accumulating data about the role of the DRB1*11 risk allele in the pathogenesis of TTP through efficient presentation of certain ADAMTS13-derived peptides by the DR11 molecule (22, 23, 42, 43), the mechanism by which the protective haplotypes lower the risk of developing TTP is to date obscure. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.