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. However, the CST complex has been shown to be involved in several aspects of telomere replication. Component of the CST complex. Morpholino knockdown in embryos results in decreased red blood cells and an arrest in T-cell progenitors, as well as increased vascularity. The vascular defects could be rescued by wild-type gene and by thalidomide treatment. Belongs to the CTC1 family.