To more accurately model CRLF2-rearranged/JAK2 mutant B-ALL, we initially generated transgenic mice to mimic the recurrent IGH–CRLF2 and P2RY8–CRLF2 translocations seen in human B-ALL (Mullighan et al. 2009a; Russell et al. 2009). The gene discussed is JAK2; the disease is precursor B-cell acute lymphoblastic leukemia.