Chromosome cohesion factor involved in sister chromatid cohesion and fidelity of chromosome transmission. Component of one of the cell nuclear antigen loader complexes, CTF18-replication factor C (CTF18-RFC), which consists of CTF18, CTF8, DCC1, RFC2, RFC3, RFC4 and RFC5. The CTF18-RFC complex binds to single-stranded and primed DNAs and has weak ATPase activity that is stimulated by the presence of primed DNA, replication protein A (RPA) and by proliferating cell nuclear antigen (PCNA). The CTF18-RFC complex catalyzes the ATP-dependent loading of PCNA onto primed and gapped DNA. Interacts with and stimulates DNA polymerase POLH. During DNA repair synthesis, involved in loading DNA polymerase POLE at the sites of local damage (PubMed:20227374). Component of the CTF18-RFC complex, which consists of CTF18, CTF8, DCC1, RFC2, RFC3, RFC4 and RFC5 (PubMed:12766176, PubMed:12930902). During assembly of the CTF18-RFC complex, CTF18 may first assemble into a subcomplex with RFC2, RFC3, RFC4 and RFC5. CTF18 then interacts directly with CTF8, which in turn interacts with DCC1 (PubMed:12766176, PubMed:12930902). The CTF18-RFC complex associates with PCNA and with DNA polymerase POLH (PubMed:12766176, PubMed:17545166). The CTF18-RFC complex does not interact with the Rad9/Rad1/Hus1 complex (PubMed:12766176). CTF18 interacts with SMC1A and RAD21 (PubMed:12930902). Interacts with DDX11 (PubMed:18499658). Associates with chromatin during S phase. Belongs to the activator 1 small subunits family. CTF18 subfamily.