Rearrangements or mutations in cytokine receptor-like factor 2 (CRLF2), located at the pseudo-autosomal region (PAR1) at Xp22.3/Yp11.3, occur in up to 7% of B-ALL cases and in almost 50% of Down syndrome-associated (DS-associated) B-ALL cases. This evidence concerns the gene CRLF2 and acute lymphoblastic leukemia.