The occurrence of childhood acute lymphoblastic leukemia (ALL) in 2 of 3 triplets provided a unique opportunity for the investigation of leukemogenesis and the natural history of ALL. The 2 leukemic triplets were monozygotic twins and shared an identical, acquired TEL-AML1 genomic fusion sequence indicative of a single-cell origin in utero in one fetus followed by dissemination of clonal progeny to the comonozygotic twin by intraplacental transfer. In accord with this interpretation, clonotypic TEL-AML1 fusion sequences could be amplified from the archived neonatal blood spots of the leukemic twins. The blood spot of the third, healthy, dizygotic triplet was also fusion gene positive in a single segment, though at age 3 years, his blood was found negative by sensitive polymerase chain reaction (PCR) screening for the genomic sequence and by reverse transcription-PCR. Leukemic cells in both twins had, in addition to TEL-AML1 fusion, a deletion of the normal, nonrearranged TEL allele. However, this genetic change was found by fluorescence in situ hybridization to be subclonal in both twins. Furthermore, mapping of the genomic boundaries of TEL deletions using microsatellite markers indicated that they were individually distinct in the twins and therefore must have arisen as independent and secondary events, probably after birth. These data support a multihit temporal model for the pathogenesis of the common form of childhood leukemia. IntroductionPediatric acute lymphoblastic leukemia (ALL) is a biologically and clinically diverse cancer. 1,2 The major subset (common ALL) has characteristics of B-cell precursors 3 ; the most frequent chromosomal and molecular genetic abnormalities are hyperdiploidy 4 and TEL-AML1 gene fusion, the latter generated by cryptic chromosomal translocation. 5,6 TEL-AML1 has transcriptional repressor function in model systems, 7,8 and it is assumed that this chimeric protein plays a key role in leukemogenesis. Until recently, however, it was unclear whether this fusion represents an initiating event in leukemogenesis and, if it does, when it occurs and what other complementary or secondary genetic changes are necessary. Studies of monozygotic twins with TEL-AML1-positive ALL have provided compelling evidence that this fusion gene can arise in utero, 9,10 validated by the finding of clonotypic TEL-AML1 fusion sequences in the neonatal blood spots (Guthrie cards) from singleton children with ALL. 11 Taken together with the concordance rate of ALL in identical twins (approximately 5%), 9 these data then suggested a multistep model for the pathogenesis of ALL involving in utero initiation and at least one necessary postnatal secondary or promotional event. 12 Deletions of the normal, nonrearranged TEL allele are frequent in patients with ALL with TEL-AML1 fusion 13,14 suggesting that this could represent a common, though not exclusive, secondary event.We describe here an unusual set of triplets in whom a monozygotic pair had ALL and their dizygotic twin brother was healthy. Leukemic cells of...