The concomitant accumulation of pro-B progenitors (247), on the other hand, increases the likelihood for illegitimate V(D)J recombination-mediated chromosomal rearrangements, in particular CRLF2 gene fusions, that can be found in approximately half of all Down syndrome BCP ALL cases (99, 104). This evidence concerns the gene CRLF2 and acute lymphoblastic leukemia.