The invention relates to cytokine receptor-like factor 2 (CRLF2), and particularly certain mutant forms of CRLF2, as prognostic and therapeutic targets in precursor B-cell acute lymphoblastic leukemia (B-ALL). Mutant CRLF2 with a Phe232-Cys (F232C) mutation is overexpressed and constitutively activates STAT5 in a subset of B-ALL patients with particularly poor prognosis. Methods and compositions useful for identifying, inhibiting expression, and inhibiting activity of the mutant CRLF2 are provided. Also provided are methods and compositions useful for treating B-ALL.
