A characteristic and baffling feature of MLL-r infant-ALL is the fact that this single oncogenic hit before birth seems to be sufficient to induce a rapidly proliferating therapy-resistant leukemia without the need for additional mutations, unlike many cases of childhood-ALL, which also originate in utero but only develop into full-blown leukemia after a second postnatal hit23. This evidence concerns the gene KMT2A and leukemia.