Although chromosomal rearrangements associated with the t(12;21) were heterogenous and complex, fusion of the sequences from chromosomes 12 and 21 on the der(21)t(12;21) chromosomes was consistent, suggesting that the TEL-AML1 gene fusion on the der(21) chromosome may be critical in leukaemogenesis and that FISH or reverse transcriptase-polymerase chain reaction (RT-PCR) targeted to the chimaeric sequences on the der(21) will be most useful in detecting the t(12;21) or following a patient with the t(12;21), which is one of the most frequent chromosomal rearrangements in both Caucasian and Asian childhood ALL.. 