JAK2 and acute lymphoblastic leukemia: Several groups discovered that murine pro-B cells expressing human CRLF2 can only drive cytokine-independent proliferation when co-expressed with ALL-associated JAK2 mutations (Mullighan et al., 2009a; Mullighan et al., 2009c; Hertzberg et al., 2010; Roll and Reuther, 2010; Yoda et al., 2010).