Components for which mutations have been identified in telomere syndromes are indicated in bold type and shaded in blue. Shelterin complex components are made up of six component proteins — telomere repeat-binding factor 1 (TRF1), TRF2, repressor/activator protein 1 (RAP1), TRF1-interacting nuclear protein 2 (TIN2), TIN2-interacting protein 1 (TPP1) and protection of telomeres 1 (POT1) — which are essential for telomere protection and for regulating telomere elongation. The telomerase enzyme complex is comprised of TERT (the reverse transcriptase) and TR (the essential RNA component that contains a template for telomere repeat addition). TR contains a 3′H/ACA box motif that binds the dyskerin protein, which is part of a larger dyskerin complex that also consists of NHP2, NOP10 and GAR1. Note that for simplicity, one dyskerin complex is shown per TR molecule, although two copies are now thought to bind each TR. Telomerase Cajal body protein 1 (TCAB1) binds a Cajal body localization motif in TR and has a role in TR trafficking and biogenesis. In the Cajal body, TR and TERT assemble into a functional holoenzyme complex. The CST complex has three components — conserved telomere protection component 1 (CTC1), suppressor of cdc thirteen 1 (STN1) and telomeric pathway with STN1 (TEN1) — which are thought to function in part in telomere lagging-strand synthesis. Figure adapted, with permission, from Ref. © (2009) Annual Reviews.