Rare cyclin D1–positive malignancies develop in Eμ^CycD1CD19^CREBim^fl/flmice. (A) Expanded spleen follicles that are diffusely positive for the B-cell marker B220 and for cyclin D1 were observed in an affected Eμ^CycD1CD19^CREBim^fl/fl mouse. (B) Multiple gastrointestinal polyps were observed in a mouse with lymphomatoid polyposis; sheets of B220 and cyclin D1–positive B cells were evident. (C) One mouse developed lung nodules that were positive for the plasma cell marker CD138 and for cyclin D1. Original magnification (left to right): (A) ×40, ×40, ×400; (B) ×40, ×200, ×200; (C) ×40, ×400, ×400.