Elimination of chromosomal instability after irradiation. (A) In fibroblasts (GM498B and GM3349B), the RDT and ABC frequencies reached normal levels three passages after irradiation, whereas the frequency of cells containing stable chromosome aberrations (S), i.e., translocations, additions, deletions, and inversions increased slightly during 10 passages; S ≤ 6% before irradiation. (B) In MLS, the RDT and ABC frequencies reached normal levels at passages 3 and 2. The OSA and MM cell lines have elevated RDT and ABC levels inherently. Irradiated OSA cells did not return to their base level until passage 8. MM reached the ABC and RDT base levels at passages 10 and 12 (not shown), respectively. The RDT values were corroborated by FISH with a probe for all human centromeres in the fibroblasts and OSA (data not shown).