Increased tumor incidence in Vk^∗MYC:Wwox KO mice with high CD138⁺ expression and uropod formation in PBPs. (A) A significantly higher number of Vk^∗MYC:Wwox KO mice (17/27) developed PBTs and BCLs compared to Vk^∗MYC:Wwox WT mice (4/14), as determined by χ² analysis (P = .036). (B) Low-magnification photomicrograph of extramedullary PBP histology section from a Vk^∗MYC:Wwox KO mouse immunostained with anti-CD138 antibody. Scale bar on lower left, 400 μm in length. (C) High-magnification photomicrograph of histology section from another Vk^∗MYC:Wwox KO PBP displaying areas of cells with immunostained uropod-like structures, that is, polarized intracellular CD138 staining to small membrane protrusions (red arrows). Scale bar on lower left, 100 μm in length. (D) Representative SPEP of serum samples from Vk^∗MYC:Wwox WT and KO mice. Red arrows indicate representative M-spikes. (E) Representative longitudinal follow-up of serum samples analyzed by SPEP collected from the same mouse through time, displaying progressive disease with a marked increase in M-spike intensity over time.