N RECENT years a number of reports describing an abnormal chromosome in patients with chronic myelocytic leukemia have appeared.1-10 This abnormal chromosome appears to belong to the group of small acrocentric chromosomes and probably represents chromosome number 21. The chromosome has been named Ph1 in relation to the city of Philadelphia where this abnormal chromosome was first described.3The mechanism of production of this abnormal chromosome has been postulated to be either a deletion or translocation of the long arms of chromosome number 21. Since chromosome number 21 is small, it is easy to detect the deletion of the long arms of the chromosome morphologically; however, it has not been possible to detect whether the deleted parts have been translocated to other larger chromosomes or simply lost, since addition of such a small segment of chromosome has been impossible to observe with the present available
GOH K, SWISHER SN. Identical Twins and Chronic Myelocytic Leukemia: Chromosomal Studies of a Patient With Chronic Myelocytic Leukemia and His Normal Identical Twin. Arch Intern Med. 1965;115(4):475–478. doi:10.1001/archinte.1965.03860160101018
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