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The article by Falletta et al. on leukemia in twins1 touches on a subject of long-standing interest to me. Some points could stand further discussion. The possibility of non-nuclear inheritance has not been mentioned by the authors. Carter2 states, "It is theoretically probable, though this has not been demonstrated, that there may be differences in cytoplasmic division in the splitting, leading to non-genetic inequalities of development." This possibility might explain some of the less than complete concordance noted in the first sentence of the abstract.