THE interpretation of chromosomal changes in human acute leukemia and the origin of such changes is complicated by two salient karyotypic findings in this disease: the remarkable diversity of the chromosomal changes, when present, in the (acute) leukemic cells; and the absence of observable chromosomal aberrations in a substantial number of cases of this disease.1 The reasons for such chromosomal diversity have remained obscure; however, studies of twins with acute leukemia might shed considerable light on the genesis of these chromosomal changes. Thus, the development of acute leukemia in a pair of infant fraternal twins offers an unusual opportunity for . . .