The location of chromosome breaks in Bloom's syndrome.

The lymphocytes of a patient with Bloom's syndrome were examined for chromosome abnormalities. The break points involved in the abnormalities in this patient were found to be nonrandom in their distribution, as was previously described in the literature in another case. It is preliminary proposed that, in both cases, many more breaks occur in chromosomal areas which replicate their DNA late in the S phase of the cell cycle than would be expected by chance. The particularly severe manifestations of the disease in this patient, including an unusually high percentage of cells with chromosome abnormalities, tend to confirm the suggestion that Bloom's syndrome may not simply be the result of one structural mutation.