Simultaneous analysis of the cell morphology and karyotypes on single colonies was carried out in two patients with Philadelphia chromosome (Ph1)-positive chronic myelocytic leukemia in blastic transformation in order to clarify the origin of leukemic cells involved. Patient no. 1 was in a typical myeloblastic transformation and patient no. 2 in “basophilic transformation.” Both patients exhibited karyotype evolution in blastic phase (BP), so that we could differentiate BP clones with additional chromosomal abnormalities from chronic phase (CP) clones with only Ph1 among single colonies. The number of single colonies yielding two or more analyzable metaphases was 18 in patient no. 1, and 19 in patient no. 2. Among these colonies, only three in patient no. 1 and none in patient no. 2 were from CP clones and 15 in patient no. 1 and 19 in patient no. 2 were from BP clones. Morphological examination revealed that not only blasts but also mature neutrophils, eosinophils, basophils, macrophages, and erythroblasts were derived from BP clones. These results suggested that (1) BP clones developed at the pluripotent stem cell level, (2) additional chromosomal abnormalities were not restricted to occur in a specific cell line representative in BP; and (3) BP clones, if not all, may retain capacity for maturation and differentiation.

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