TEN1 is a critical component of the CST (CTC1-STN1-TEN1) complex that functions as a specialized replication factor essential for telomere maintenance and genome stability 1. The protein plays dual roles in telomere biology: it promotes telomere duplex replication and C-strand fill-in synthesis while also regulating telomere length homeostasis 1. TEN1 is recruited to telomeres through interactions with the POT1/TPP1 shelterin complex, where phosphorylated POT1 holds CST-Polα/primase in an autoinhibited state until telomerase completes G-strand extension 2. Beyond telomeres, TEN1 functions in genome-wide replication restart following fork stalling and contributes to DNA double-strand break repair pathway choice by suppressing DNA end resection 13. Loss of TEN1 function has significant clinical relevance, as demonstrated in mouse models where Ten1 deficiency causes telomere shortening, accelerated aging, aplastic anemia, and cerebellar hypoplasia - phenotypes resembling human dyskeratosis congenita 4. Pathogenic variants in TEN1 and its interacting partners are associated with telomere biology disorders, highlighting its crucial role in maintaining chr17 stability 5. The protein also cooperates with RECQ4 helicase to resolve G-quadruplex structures that can impede replication 6.