Sister chromatid exchange in normal adults long after thymus irradiation.

Chromosomal breaks are seen in the peripheral blood lymphocytes of irradiated patients long after the original exposure. This is confirmed in our present investigation. We found 141 (20%) chromosomal breaks in 707 metaphases from 18 thymus-irradiated patients and only 26 (6.3%) in 410 metaphases from six of their normal siblings. Sister chromatid exchange (SCE) has been used to test the mutagen-carcinogen agents. However, radiation, a well-known mutagen-carcinogen agent, has no effect upon SCE. This suggests that the SCE and chromosomal breakage mechanisms may be quite different; study of them may help us to understand chromosomal breakage and healing.