IN 1960, Nowell and Hungerford demonstrated an abnormal chromosomal marker in chronic myelocytic leukemia (CML).1 This observation has been confirmed2-8 and the abnormal chromosome has been named Philadelphia or Ph1 chromosome.3 The mechanism of its production may be analogous to the production of a smaller G choromosome found in patients accidentally exposed to total-body irradiation.9 If so, it would indicate that the Ph1 chromosome is an acquired abnormality rather than a prezygotic abnormality. This postulation was supported by our finding that there was no Ph1 chromosome in a normal identical twin whose leukemic twin showed typical Ph1 chromosome.10 This communication reports the clinical and cytogenetic findings in another set of identical twins, one having CML; it also reports follow-up studies of the surviving normal identical twin of our first reported set10 7½ years after the diagnosis of CML in his
Goh K, Swisher SN, Herman EC. Chronic Myelocytic Leukemia and Identical Twins: Additional Evidence of the Philadelphia Chromosome as Postzygotic Abnormality. Arch Intern Med. 1967;120(2):214–219. doi:10.1001/archinte.1967.00300020086011
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