NFKB1 and Splenomegaly: IgM+ lymphoproliferation and, in at least two cases with serum IgM M-spikes, incipient LPL-like neoplasms arose in BCL2+IL6+AID− mice with full genetic penetrance (100% incidence) and relatively short latency (~3-month survival), progressed with a predominantly extra-medullar growth pattern (splenomegaly and lymphadenopathy) and exhibited molecular genetic traits (MYD88/NFκB activation) that overlapped with human WM.