Cytogenetic studies were performed in 331 patients with ALL diagnosed at the National Institutes of Health between January 1961 and January 1976. Four patients had constitutionally abnormal genotypes, three had Down's syndrome, and one had a D/G translocation. Aneuploidy was observed in the pretreatment bone marrow in 49/115 (42.6%) of the patients and at some stage of disease in 54.1% of the patients. Aneuploidy in this series exhibited several general characteristics: aneuploid cells usually coexist with normal stem cells, hyperdiploidy is predominant, and wide ranging aneuploidy clusters around a major cell line. The most common chromosomal group involved in aneuploidy is the G group (p = 0.001) and the next most common is the B group (p = 0.01). Aneuploidy disappeared after successful achievement of remission, and new clones developed in 12 patients during relapse. Two of the four patients originally thought to have a Ph1 chromosome, on trypsin Giemsa banding were proved to have a 21q‐ chromosome. A higher incidence of aneuploidy was noted in patients under one year or more than 20 years of age and was also higher in patients with low or elevated WBCs at diagnosis. The appearance of aneuploid cells in the bone marrow at the onset or later in the disease is of no prognostic significance but persistence of these lines and the development of total aneuploidy signals a poor prognosis. Eradication of aneuploid cells is therefore essential for the achievement of a long remission and progress to a permanent cure.
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