Gain-of-function mutations in JAK2 have been identified in the high-risk Ph-like ALL subtype, occurring exclusively with rearrangements of CRLF2 (CRLF2r), which lead to CRLF2 overexpression (Roberts et al., 2012; Roberts et al., 2014a; Boer et al., 2017; Tasian et al., 2017b; Pui et al., 2017; Reshmi et al., 2017; Steeghs et al., 2017). Here, CRLF2 is linked to acute lymphoblastic leukemia.