The ABO mismatch driven hemolysis can present with (1) immediate intravascular hemolysis mediated by host ABO antibodies directed against donor RBCs in the graft; (2) delayed hemolysis from residual host cells reacting to RBCs produced by the engrafted donor marrow, and (3) pure red cell aplasia (PRCA). The gene discussed is ABO; the disease is Pure red cell aplasia.