Hofer, K. G., Lin, X. and Schneiderman, M. H. Paradoxical Effects of Iodine-125 Decays in Parent and Daughter DNA: A New Target Model for Radiation Damage. Chinese hamster ovary cells were synchronized at the G 1 /S-phase boundary of the cell cycle and were pulse-labeled with 125 I-iododeoxyuridine 30 min after they entered the S phase. Cell samples were harvested and frozen for accumulation of 125 I decays during the first and second G 2 phase after labeling. Cell aliquots that had accumulated the desired number of decays were thawed and plated for evaluation micronucleus formation and cell death. Cells subjected to 125 I decays during the first G 2 phase after labeling exhibited single-hit kinetics of cell killing ( n = 1, D 0 41 decays/cell). In contrast, decays accumulated during the second G 2 phase killed cells with dual-hit kinetics ( n = 1.9, D 0 81 decays/cell). A similar divergence in the action of 125 I was noted for micronucleus formation. These findings indicate that the effects of 125 I varied depending on whether the decays occurred in daughter DNA (first G 2 phase) or parent DNA (second G 2 phase). Control studies with external X rays showed no such divergence of the action of radiation. To account for this paradox, a model is proposed that invokes higher-order chromatin structures as radiation targets. This model implies differential spatial arrangements for parent and daughter DNA in the genome, with DNA strands organized such that a single 125 I decay originating in daughter DNA damages two targets during the first G 2 phase, but identical decays occurring during the second G 2 phase damage only one of the targets.