Determination of genomic instability in the PT1-CHO cell lines. a Examples of chromosomal and chromatin abnormalities observed after DAPI-staining. Shown are those seen from PT1-30; indicated by arrow(s): (1) chromatin bridges at anaphase; (2) a lagging chromosome at late anaphase; (3) a micronucleus at telophase; (4) a micronucleus beside a smaller one; (5) nuclear buds; (6) chromatin condensation/fragmentation. b Frequencies of micronuclei, nuclear buds, chromatin condensation and ana/telophase abnormalities, showing higher frequencies for PT1-1 than the other three cell lines. One-way ANOVA tests for micronucleus formation (*P = 0.0457), and ana/telophase abnormalities (*P = 0.0221) were found significant. Cells were grown in 1-well chamber slides. The frequencies for micronucleus, nuclear bud, and chromatin condensation/fragmentation were determined from n = 4000 cells. The frequency for abnormal mitotic stages were determined in n = 100 ana/telophases. Data represent means and standard error of the means (SEM) of measurements of two passages (P55 and P72).