To investigate whether the coexpression of Sf3b1 mutation accelerates CLL in Mdr MT mice, we monitored the onset of CLL in 3 mouse cohorts, namely mice having B cell–specific homozygous or heterozygous deletion of Mdr with (DM, n = 25) or without (Mdr MT, n = 27) heterozygous Sf3b1-K700E, or lacking 2 lesions (WT, n = 30). This evidence concerns the gene SF3B1 and B-cell chronic lymphocytic leukemia.