The majority of cases in Ph-like B-ALL harbors CRLF2 gene rearrangements (either IGH–CRLF2 or P2RY8–CRLF2) leading to cell surface overexpression of CRLF2, a component of the heterodimeric cytokine receptor complex for thymic stromal lymphopoietin (TSLP) (Mullighan et al. 2009a; Russell et al. 2009). Here, CRLF2 is linked to acute lymphoblastic leukemia.