Therefore, we reasoned that the elongation of the green phase duration after irradiation simply reflects G2 arrest. Therefore, we speculated that cells carrying a wild-type p53 gene with Fucci probes may exhibit elongation of red phase, representingG1 arrest. Because appropriate regulation of p53 expression was very difficult in this system, we performed further analysis using BJ1-hTERT-Fucci cells, which were established from h-TERT�Cimmortalized normal human diploid foresk in fibroblasts with wild-type p53 function. In an unirradiated condition, green cells entered M phase first, whereas red cells entered M phase after almost all the green cells had done so, like Fucci-HeLa cells. After 5 Gy irradiation, most cells irradiated in red phase exhibited are markable elongation of red phase, and cells irradiated in green phase turned red before the irradiated red cells entered M phase. Thus, the catching up of red cells with green cells, which was characteristic of HeLa Fucci cells after irradiation, was unlikely to occur in cells carrying a functional p53 gene. To date, analysis of the effects of irradiation at different cell-cycle phases on subsequent cell kinetics has required isolation of synchronized cell populations by artificial methods. Indeed, since the comprehensive study by Terasima and Tolbutamide Tolmach using the shake-off method about 50 years ago, technical limitations have AS-604850 prevented further substantive progress in this field. The emergence of the Fucci system allowed us, for the first time, to analyze these phenomena in an asynchronous cell populations. Because HeLa-Fucci cells have non-functional p53 and donotexhibit G1 arrest, we expected that observations using this system could focus on elongation of S andG2 phases. The length of the green phase detected using the Fucci system is a useful indicator of cell-cycle kinetics, including G2 arrest, because cells in both S and G2 phases emit green fluorescence. However, given that endoreduplication occurs in p53-deficient cells following DNA damage, the situation becomes much more complicated.