Complex chromosomal abnormalities in utero , 5 years before leukaemia *

Summary Prenatal acquisition of leukaemia‐associated gene rearrangements is a well‐established phenomenon. This is the first report of a complex cytogenetic clone, in association with an ETV6/AML1 fusion, developing in utero . Identical twin girls, aged 4 years, developed ETV6/AML1 ‐positive acute lymphoblastic leukaemia (ALL) within 3 months of one another. Both demonstrated an identical four way, variant t(12;21). There was gain of an AML1 signal in twin 1 and loss of an ETV6 one in twin 2 at interphase. This unique case study demonstrates that ETV6/AML1 fusion and the associated complex chromosomal rearrangements occurred in utero . Clonal expansion of the abnormal cell in one twin was followed by metastasis to the other. There was a prolonged preleukaemic phase, which lasted well into childhood. The short time between the two diagnoses of ALL suggests a common precipitating event. The significance of the different secondary markers remains unclear.