Genotoxic stresses such as DNA damage or abnormal chromosome segregation induced by various extrinsic or intrinsic factors often take place in all kinds of cells, which play important roles in tumorigenesis. Organisms have developed many molecular mechanisms regarding these genotoxic stresses during the evolutional progress. Protein post-modifications such as phosphorylation or sumoylation have been recognized as the basis of protein functions (see the related papers in No. 5 issue, 2011), they also involve in the regulation of all types of genotoxic stresses. The collection of five research articles in this issue provides the new data how protein modifications control or respond to the various genotoxic stresses.