During its duplication, DNA, the carrier of our genetic information, is particularly vulnerable to decay, and the capacity of cells to deal with replication stress has been recognised as a major factor protecting us from genome instability and cancer. A major pathway that allows cells to overcome or bypass DNA lesions during replication is activated by posttranslational modifications of the sliding clamp protein PCNA. Whereas monoubiquitylation of PCNA allows mutagenic translesion synthesis by damage-tolerant DNA polymerases, polyubiquitylation is required for an error-free pathway that involves template switching to the undamaged sister chromatid, involving a recombination-like mechanism. Hence, damage bypass contributes to genome maintenance, but can itself be a source of genomic instability. It is therefore not surprising that PRR is a highly regulated process whose activity is limited to the appropriate situations by stringent control mechanisms.The proposed project aims at understanding DNA damage bypass in its cellular context. Using a combination of new and established technology, we will address the role of chromatin dynamics in the reaction, its spatial and temporal control in relation to genome replication, and its coordination with other PCNA-dependent processes in the cell. To this end, we will establish technology to isolate and analyse the composition of damage bypass tracts, develop and implement novel methods to induce lesions and image damage processing in live cells, and exploit a spectrum of biochemical and biophysical techniques to investigate the role of PCNA as a molecular tool-belt in the coordination of its interaction partners. In combination, these approaches will give important insight into how the replication of damaged DNA is managed with high efficiency and accuracy within the cell.