Tandem duplication q14 and dicentric formation by end-to-end chromosome fusions in ataxia telandiectasia (AT). Clinical and cytogenetic findings in 5 patients.

Chromosome studies on lymphocyte cultures were performed in 5 patients with AT, 2 of whom had been followed for 4 years. Four out of these patients showed an increased incidence of chromosome-type aberrations. A clonal development was present in one patient, 96% of his metaphases containing a tandem duplication of almost the entire long arm 14. Four years earlier the proportion of these cells was 80%. Two other patients presented a small proportion of cells with an unidentified abnormally long D chromosome. In a total of 724 metaphases from 4 patients 31 dicentric chromosomes were observed, all of a peculiar type; in their formation no chromosome material was lost and they all seem to have arisen by end-to-end fusions. The incidence of chromatid-type aberrations was normal or at the upper limit of control values in all 5 cases. The sister chromatid exchange rate studied with BUDR in 3 patients was found to be normal.