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. In addition to telomere protection, the CST complex has probably a more general role in DNA metabolism at non-telomeric sites (By similarity). Component of the CST complex, composed of TEN1, CTC1 and STN1. Interacts with TEN1 and CTC1; the interaction is direct. Interacts with ACD/TPP1 (By similarity). Belongs to the STN1 family.