We have isolated eight PALA-resistant mutants from CHO-PV cells and have shown that the CAD gene was amplified. We then localized the CAD genes with fluorescence in situ hybridization followed by G-banding and identified 10 different marker chromosomes carrying amplified DNA. TTAGGG repetitions, which normally map to the telomeres and centromeres, have also been localized on the 10 marker chromosomes. The organization of amplified genes and of TTAGGG sequences suggests that dicentrics were formed during amplification and that breakage-fusion-bridge cycles may have generated 7 marker chromosomes. One isochromosome was probably derived from abnormal centromere segregation at anaphase. The most striking observation was that TTAGGG sequences of centromeric origin surrounded the amplified regions and were always localized at the telomeres of the chromosome arms carrying amplified DNA. These results indicate that the recombination events that accompanied gene amplification frequently involved centromeric DNA. Moreover, breakage within centromeric TTAGGG repeats may produce telomere-like structures that stabilize the ends of rearranged chromosomes.