Ataxia telangiectasia (AT) is a human autosomal recessive disorder in which patients show a marked predisposition to malignant disease and cytogenetic abnormalities. We report here the levels of spontaneously occurring chromosome aberrations and particularly the presence of cytogenetically marked clones of cells in peripheral lymphocytes of 13 patients. There is a variation between the patients with respect to frequency of different aberration types, and clones are present in 5/13 patients. Several of these patients appear to have more than a single clone, possible clones or subclones. There is no evidence for any malignant disease in any of these patients. A description is given from one of these patients, of the most complex clone so far reported in an AT patient without malignant disease. The development of such a complex clone might be important as a step in malignant change. Similarities between this clone and one reported in an AT patient with T-cell chronic lymphocytic leukaemia are discussed.

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Clones of cytogenetically abnormal cells have been recognized in fibroblasts cultured from normal human adult skin. No such clones have been observed in human embryo skin fibroblasts cultured in the same way. Although the culture conditions may have played some part in the emergence of these clones, it is possible that the abnormal cells from which the clones were derived were present in vivo.