Acts in a DNA repair pathway for removal of UV-induced DNA damage that is distinct from classical nucleotide excision repair and in repair of ionizing radiation damage. Functions in homologous recombination repair of DNA double strand breaks and in recovery of stalled replication forks. Component of the Smc5-Smc6 complex which consists of KRE29, MMS21, NSE1, NSE3, NSE4, NSE5, SMC5 and SMC6. Interacts with NSE3. Belongs to the NSE4 family.