epidemiologic associations with radiation or viral exposures during pregnancy [6].
Concordant leukemia has been reported in monochorionic and dichorionic infant twins, each pair with a clonal, nonconstitutional rearrangement of MLL gene. In addition, a pair of identical twins, aged 9 and 11 years at diagnosis, showed the same clonal T cell malignancy. The prenatal origin of leukemia has been directly demonstrated in three patients with 4;11. Moreover, two twins with ALL and t(12;21), aged 3, 5, and 5 years at diagnosis, showed the same sequence of the ETV6-AML1 fusion could be originated in utero, from data presented by Cazzaniga and Biondi at the 10th Annual Meeting of the International BFM Study Group in 1999.
Genetic mapping studies have led to the identification of the first molecular correlate of human non-disjunction; i.e., altered levels and positioning of meiotic recombination events have been observed in all human trisomic conditions yet studied. These observations have led to the idea that human meiotic non-disjunction requires ''two hits'': first, the establishment in prophase I of a ''vulnerable'' bivalent and second, abnormal processing of the bivalent at metaphase I or II [7].
Recent research on the mechanism of action of the cellular components that segregate chromosomes accurately during mitosis or meiosis has served to highlight the number of potential targets for disruption. The process of chromosome segregation represents an orchestrated chain of events centred on the activities of cellular kinesins and dyneins. These ''motors'' are involved in arranging chromosomes at the metaphase plate, providing the spindle tension necessary for progression, and the actual segregation of the chromosomes to the poles [8][9][10][11][12][13][14][15][16][17][18].
In this case, there were two changes or two ''hits,'' one that led to mutation in WSCR1 and the other that led to made ALL with hyperploidy. Both ''hits'' could have happened in utero.
To the Editor: Congenital fibrosarcoma (FS) is a rare softtissue malignant tumor that usually shows up as a mass affecting the distal portions of the upper and lower extremities, often misdiagnosed at birth as hemangioma. This tumor is histologically identical to the adult-type fibrosarcoma, although the