Anomalous effects of 125I after its incorporation into mammalian cell DNA.

During labeling of mammalian cells with 125I-iododeoxyuridine for studies on single-strand break induction and repair, care must be taken to keep the amount of 125I incorporated per cell to very low levels. Under some conditions enough 125I can be incorporated during the incubation period (generally about one generation time) to damage the repair systems of cells so extensively, even before they are frozen, that they cannot repair any of the breaks induced by the 125I during the time they are frozen to accumulate these breaks.