Although there are no accurate data for concordance rates of leukemia in infant twins, anecdotally it seems to be exceptionally high, perhaps approaching one hundred percent that is, if one twin has it, unfortunately so will the other. If correct, this suggests that MLL gene fusion in utero has a dramatic impact, ensuring subsequent leukemia. But for children aged two to six years with acute lymphoblastic leukemia, the concordance rate is considerably lower at around five percent. This still represents a one hundred fold extra risk of leukemia for the twin of a patient with acute lymphoblastic leukemia but also indicates the need for some additional postnatal event for which there is a one in twenty chance, or ninety five percent discordance. This suggests, at a minimum, a “two hit” model for the natural course of childhood leukemia.
If this model of leukemia development is correct, then, for every child with acute lymphoblastic leukemia diagnosed, there should be at least twenty healthy children who have had a chromosome translocation, a functional leukemia fusion gene, and a covert preleukaemic clone generated in utero. This possibility has been investigated by screening unselected samples of newborn cord blood for fusion genes. About six hundred samples have been screened, and around one percent have a leukemia TEL