Figure 5. Micronuclei in response to DNA damage are increased during S phase. HME mouse fibroblasts and WI38 human fibroblasts were synchronized in G1 (A) or S phase (B). Both cell types were then subjected to DNA damage by treatment with neocarzinostatin (NCS) for two hours. Cells were fixed at 48 hours or 72 hours following NCS treatment, and the number of cells containing at least one micronucleus was counted. Images were analyzed using Slidebook software, and an average micronuclei count was taken from at least 400 cells per treatment and time point. Non-damaged HME and WI38 fibroblasts were included as controls. Differences between mouse and human cells statistically significant at P<0.01 are marked with two asterisks.