To determine if mice expressing the Q72L mutant of Rras2 also developed CLL, we crossed knock-in Rras2(Q72L)fl/fl mice bearing a repeated and inverted exon 3 in the Rras2 locus with mb1-Cre mice to specifically exchange wild-type exon 3 for the mutant exon in B cells. This evidence concerns the gene RRAS2 and B-cell chronic lymphocytic leukemia.