The observation of damage to chromosomes and of alterations to normal cell cycle progression were early findings in radiation biology (1 –6 ) and provided a strong impetus for the elaboration of the causative basis for understanding deoxyribonucleic acid (DNA) repair in all its manifestations. See Murnane (7 ) for a historical perspective. Ionizing radiations provide a precise tool in this regard, in that they are readily characterized physically, can be delivered with relative precision as well as tight control of timing, and do not require metabolic activation. Such radiations produce discrete energy-deposition events in cells, the intensity and frequency of which depend on the dose and type of radiation. The charged particles that are produced—electrons after photon (X- and γ-ray) irradiation; protons, with heavier charged particles, after neutron irradiation; and α-particles after isotopic or machine irradiation—deposit their energy along relatively short tracks, often of cellular dimensions.
