Component of the CST complex proposed to act as a specialized replication factor promoting DNA replication under conditions of replication stress or natural replication barriers such as the telomere duplex. The CST complex binds single-stranded DNA with high affinity in a sequence-independent manner, while isolated subunits bind DNA with low affinity by themselves. Initially the CST complex has been proposed to protect telomeres from DNA degradation (PubMed:19854130). However, the CST complex has been shown to be involved in several aspects of telomere replication. The CST complex inhibits telomerase and is involved in telomere length homeostasis; it is proposed to bind to newly telomerase-synthesized 3' overhangs and to terminate telomerase action implicating the association with the ACD:POT1 complex thus interfering with its telomerase stimulation activity. The CST complex is also proposed to be involved in fill-in synthesis of the telomeric C-strand probably implicating recruitment and activation of DNA polymerase alpha (PubMed:22748632). The CST complex facilitates recovery from many forms of exogenous DNA damage; seems to be involved in the re-initiation of DNA replication at repaired forks and/or dormant origins. Required for efficicient replication of the duplex region of the telomere. Promotes efficient replication of lagging-strand telomeres. Promotes general replication start following replication-fork stalling implicating new origin firing. May be in involved in C-strand fill-in during late S/G2 phase independent of its role in telomere duplex replication (By similarity). Component of the CST complex, composed of TEN1/C17orf106, CTC1/C17orf68 and STN1; in the complex interacts directly with TEN1 and CTC1 (PubMed:19119139, PubMed:19854130). Interacts with ACD/TPP1 (PubMed:19648609). Interacts with POT1 and POLA1 (By similarity). Belongs to the STN1 family. According to PubMed:19119139, it acts as a regulator of DNA replication. According to PubMed:19854130, this effect is indirect and it rather acts as a general regulator of DNA metabolism.