JAK2 and precursor B-cell acute lymphoblastic leukemia: Approximately 50% of CRLF2-rearranged B-ALLs have concomitant activating point mutations at highly conserved regions of JAK2 (exon 16), the most common of which is the B-ALL-exclusive activating JAK2R683G alteration (Mullighan et al. 2009a; Harvey et al. 2010; Hertzberg et al. 2010).