Schematic diagram illustrating the hypothesized mechanism of 'closed chain' rearrangement formation. Chromatin structure or transcription factor binding causes certain chromosomes to be physically near each other. A genomic insult such as ionizing radiation may disrupt these co-localized chromosomes simultaneously. DNA repair machinery can incorrectly join chromosome fragments, resulting in chimeric chromosomes containing disruptions in multiple genes. The figure is adapted from Figure 2A in Berger et al. .