Identical twins and their older brother have been studied: all diagnosed within a span of three months as having chronic myelocytic leukemia; both twins were symptomatic and the brother asymptomatic. Chromosome analyses were made on peripheral blood cells. The asymptomatic brother had never been treated. The Ph1 chromosome was present in each of the three siblings.

A number of other chromosome abnormalities were found. Results were interpreted in terms of a probable genetic factor in the Philadelphia chromosome and susceptibility to chronic myelocytic leukemia.

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