The critical secondary events leading to leukemic transformation in t(12;21)/ETV6:RUNX1 BCP-ALL would frequently be linked to genomic rearrangements mediated by aberrant RAG recombinase activity (27), which is increased in t(12;21)/ETV6:RUNX1 BCP-ALL (40, 41). This evidence concerns the gene RUNX1 and acute lymphoblastic leukemia.