Deregulated overexpression of CRLF2 (Δ–CRLF2), found exclusively in 5–10% B-ALL cases without known molecular rearrangements56,57 is usually sustained by two types of aberrations: a rearrangement that involves CRLF2 and the Ig heavy chain locus (IGH@-CRLF2) or an interstitial PAR1 deletion that juxtaposes intron 1 of P2RY8 to the coding region of CRLF2 itself. This evidence concerns the gene CRLF2 and precursor B-cell acute lymphoblastic leukemia.