Given that there are at least two putative human fetal ELP subsets based on CD127 expression, it remains to be determined if both fetal ELP subsets are affected by the KMT2A/AFF1 fusion gene, or whether the leukemic/preleukemic event occurs in another progenitor type that then arrests at the ELP stage, and, if so, if this has any bearing on the development of infant versus childhood ALL. The gene discussed is AFF1; the disease is acute lymphoblastic leukemia.