Differential repair activity of human chromosomes

The frequency of radioactive label (3H-thymidine) incorporation in human lymphocyte chromosomes in the course of repair (extra) synthesis of DNA was assessed. Chromosomes 7, 12, and 21 were found to have a lower, and chromosome 22 a higher repair activity than was expected when a uniform incorporation of the label along the genome was hypothesized. Segments were detected that incorporated an increased number of labels in the course of extra DNA synthesis.