CD4 and Splenomegaly: In addition, 6/28 mice exhibited splenomegaly accompanied by the accumulation of CD19+IgMhiCD5hiB220int B1 cells consistent with the development of B‐cell leukaemias (Fig 1B), whereas CD3‐negative CD4+CD8+ double‐positive cells reminiscent of T‐cell leukaemic blasts (FSC‐Ahi) were evident in the thymi or peripheral lymphoid organs of 5/28 mice (Fig 1A and B; Appendix Table EV1).