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Blood lymphocytes from a boy with Bloom’s syndrome and a man with a mild form of xeroderma pigmentosum were examined for their response to UV-irradiation, as measured by the levels of induced unscheduled DNA synthesis, and to X-irradiation, as measured by chromosome aberrations. Although the individual with Bloom’s syndrome showed facial skin sensitivity to sunlight, his lymphocytes showed a normal response in unscheduled DNA synthesis. In contrast, cells from the individual with xeroderma pigmentosum showed a 50 % reduction in the amount of unscheduled DNA synthesis following UV exposure. Cytogenetic studies on PHA-stimulated Bloom’s syndrome lymphocytes revealed a typical increase in the incidence of aberrations. Many of the Bloom’s cells showed breakages located at the centromere and equal, symmetrical chromatid interchanges involving homologous chromosomes; these aberrations were not produced in control cells cultured in serum from the Bloom’s syndrome patient. In contrast, the frequency of spontaneous aberration in xeroderma pigmentosum lymphocytes was similar to that in controls. G₁ cells from both patients were exposed to a range of X-ray doses, and it was shown that neither cell type differed from controls in the incidence of induced chromosome-type aberrations. The significance of these findings is discussed.

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The finding of an inherited chromosome abnormality (Chl) in several members of a family, including two who had developed chronic lymphocytic leukaemia (Gunz, Fitzgerald, and Adams, i962), led to the suggestion that this abnormality predisposed its carriers to the development of the disease, and that inherited cytogenetic abnormalities of this type might determine some instances of familial leukaemia. In an attempt to test this hypothesis we have now made chromosome investigations of a further ii families with leukaemia, or leukaemia and a related neoplastic disorder, in two or more first-degree relatives. Methods In families showing a multiple occurrence of leukaemia and related disorders, we examined the chromosomes of all surviving individuals with leukaemia and sometimes of their normal relatives. If no chromosome abnormality was found in the leukaemic individuals, no further members of the family were examined, the justification of this approach being that an inherited chromosome abnormality causing increased risk of leukaemia in a family should normally be present in the leukaemic members. The chromosome examinations were made on blood leucocytes cultured with phytohaemagglutinin according to the method of Moorhead, Nowell, Mellman, Battips, and Hungerford (I960), with minor modifications. Although in some individuals the cultured cells may have consisted of a mixture of normal and leukaemic lymphocytes, this was not considered to detract from the usefulness of the method because the inherited cytogenetic abnormalities being investigated would be expected to occur in all cells of a carrier. The chromosomes were examined in approximately 30 metaphase figures from each individual, particular attention being given to gross abnormalities in chromo

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