Leukaemia and childhood cancer in twins.

Data from the United Kingdom on childhood leukaemia and childhood cancer in twins and from the United States on leukaemia in twins are analysed by a new method. The method distinguishes determinants occurring before the stage of pregnancy corresponding with the cleavage of MZ pairs, from determinants occurring after this point. It derives estimates of the frequencies of each class of determinant. Different aetiological models are characterised by particular combinations of frequencies, and can thus be identified. The results of the analysis suggest that the major determinants of childhood leukaemia, and possibly of the solid cancers as well, operate before the time of cleavage. They operate either on the early zygote or its component germ cells. These early determinants are not, however, sufficient causes and require combination with postcleavage determinants, which subsequently occur in about a quarter of all children, before leukaemia can ensue.