SUMO modification (SUMOylation) is a postranslational modification playing an important role in chromatin structure, DNA metabolism, transcriptional regulation and genomic stability. Molecular mechanisms underlying such effects of SUMO-modified proteins are largely obscure, but the influence of SUMOylation on genomic stability seems to be of particular importance since its disturbances may lead to serious pathological states, including cancer. Cell cycle control and DNA repair proteins essential for preserving genomic stability and SUMO modification may change their localization and function. We have relatively much information on some subnuclear components, like nucleous, while little is known on some others, like PML bodies, but the results of some recent research suggest, that this is SUMOylation, which plays a key role in their architecture and functioning. The influence of SUMO modifications on genomic stability and chromatin structure may be fundamental for cell's viability, therefore the mechanisms of SUMOylation are worth studying.