Plays a role in the DNA damage response (DDR) pathway by regulating postreplication repair of UV-damaged DNA and genomic stability maintenance. The SLF1-SLF2 complex acts to link RAD18 with the SMC5-SMC6 complex at replication-coupled interstrand cross-links (ICL) and DNA double-strand breaks (DSBs) sites on chromatin during DNA repair in response to stalled replication forks. Promotes the recruitment of SLF2 and the SMC5-SMC6 complex to DNA lesions. Interacts (via BRCT domains) with RAD18 (via C-terminus and phosphorylated form); this interaction is required for efficient repair of UV-induced DNA damage (PubMed:15632077, PubMed:22036607). Interacts (via N-terminus) with SLF2; this interaction links RAD18 to the SMC5-SMC6 complex. Interacts (via BRCT domains) with RAD18; this interaction occurs in a SLF2-independent manner. Interacts with SMC6. Relocalizes with RAD18 to nuclear foci in response to DNA damage (PubMed:22036607). Colocalizes with RAD18 in the nucleus and to centrosomes (PubMed:15632077). Associates with chromatin. Accumulates with RAD18 and the SMC5-SMC6 complex at replication-coupled DNA interstrand repair and DNA double-strand breaks (DSBs) sites on chromatin in a ubiquitin-dependent manner. Widely expressed (PubMed:15632077). Expressed in testis (PubMed:15632077). Expressed in spermatocytes (PubMed:15632077). Expressed in the developing embryo (PubMed:15632077). BRCT domains are necessary for its targeting to ionizing radiation-induced nuclear foci (PubMed:22036607). Mice develop normally, display no pathological abnormalities and are fertile (PubMed:15632077).