Component of the CST complex, a complex that binds to single-stranded DNA and is required to protect telomeres from DNA degradation. 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 (PubMed:19064932). Associates with enzymatically active telomerase (PubMed:25329641). Plays a genomewide role in DNA replication and facilitates re-replication at non-telomeric loci (PubMed:25299252). Component of the CST complex, composed of CTC1, TEN1 and STN1. Interacts with CTC1 (PubMed:19854131). Interacts with TEN1 (PubMed:23572541, PubMed:25329641). Interacts with POT1A (PubMed:25329641). In vitro interaction with TEN1 and POT1A is mutually exclusive, indicating that POT1A and TEN1 may compete for the same binding site (PubMed:25329641). Widely expressed. Plants display an immediate onset of growth and developmental defects and reduced fertility, probably due to impaired telomeres (PubMed:19064932). In nearly all mutants, apical dominance is completely abolished, leading to multiple inflorescence bolts that are often fused (PubMed:19064932). In addition, floral phyllotaxy is perturbed and siliques develop at irregular positions on the inflorescence bolt (PubMed:19064932). Moreover, leaf size is substantially reduced, likely reflecting defects in cell proliferation (PubMed:19064932). These effects are accompanied by catastrophic loss of telomeric and subtelomeric DNA, high levels of end-to-end chromosome fusions, increased G-overhang signals, and elevated telomere recombination (PubMed:19064932). Progressive loss of telomeric DNA and gradual onset of telomere dysfunction (PubMed:25299252). Hindered re-replication of heterochromatic regions (PubMed:25299252). Belongs to the STN1 family.