LY9 and systemic lupus erythematosus: The role of Ly9 as an inhibitor molecule in the development of spontaneous autoimmunity, excluding any effects dictated by epistatic interactions, is further supported by the observation that 12-month-old Ly9−/−(BALB/c.129) mice exhibited splenomegaly, which is a feature often present in SLE-prone mice (32, 33).