Mutations of CRLF2 in patients with Down syndrome–associated acute lymphoblastic leukemia. (A) Example of sequences depicting the F232C in CRLF2. The F232C (arrowed) is present at diagnosis but not in remission. The wild-type sequence denotes positions of both nucleotides and amino acids. (B) Expression of CRLF2 F232C mutation. Examples of 2 patients: in one (i) both alleles, wild-type and mutated, are expressed, whereas in the other (ii) only the mutated allele is expressed. (C) Schematic presentation of CRLF2. SP indicates signal peptide; EC, extracellular region; TM, transmembrane region; and Cy, cytoplasmic region. Numbers indicate amino acid position. (D) Cytokine withdrawal assay of BaF3 cells stably expressing wild-type mouse FLAG-Jak2 that were transduced with either wild-type human CRLF2 or human CRLF2 F232C. Error bars represent SE. (E) Constitutive activation of the Jak/Stat5 pathway in BaF3 cells expressing wild-type mouse FLAG-Jak2 and either wild-type human CRLF2 (wt) or human CRLF2 F232C (F232C), after 5 hours of cytokine deprivation.