 In addition to S-phase checkpoints, G1 and G2 checkpoints exist to check for transient DNA damage which could be caused by mutagens such as UV damage.An example is the Saccharomyces pombe gene rad9 which arrests the cells in late S/G2 phase in the presence of DNA damage caused by radiation.The yeast cells with defective rad9 failed to arrest following radiation, continued cell division and died rapidly while the cells with wild-type rad9 successfully arrested in late S/G2 phase and remained viable.