TEL-AML1 gene fusion appears to be an initiating event and usually occurs before birth, in utero.     ///    "Origins of ""late"" relapse in childhood acute lymphoblastic leukemia with TEL-AML1 fusion genes.. "    ///    " Approximately 20% of childhood B-precursor acute lymphoblastic leukemia (ALL) has a TEL-AML1 fusion gene, often in association with deletions of the nonrearranged TEL allele. "    ///    "It is speculated that, in some patients, combination chemotherapy for childhood ALL may fail to eliminate a fetal preleukemic clone with TEL-AML1 and that a second, independent transformation event within this clone after treatment gives rise to a new leukemia masquerading as relapse. "    ///    "In contrast, a patient who had a relapse while on treatment with TEL-AML1 fusion had essentially the same TEL deletion, though with evidence for microsatellite instability 5(') of TEL gene deletion at diagnosis, leading to extended 5(') deletion at relapse. "