NR4A2 and Splenomegaly: Male BXSB.Yaa mice developed severe splenomegaly by 16–20 weeks of age similar to wild‐type male BXSB.Yaa mice, while BXSB.Yaa Nr4a2 cKO littermates were significantly protected from lymphoid expansion (Figs 1A–C and EV1A).