and hinge domains Two other domains also work to bind and influence the activity of proteins associated with the TERF2 protein. Both are unique to TERF2. The basic domain sits at the N-terminal, and has two main functions: the prevention of t-loop excision by XRCC3, and the inhibition of SLX4. The final domain of TERF2 is called the hinge domain (). This domain contains a motif for binding the shelterin protein TIN2, which acts as a stabilizing protein, connecting units that are attached to double stranded and single stranded DNA. This domain also is responsible for binding to RAP1, and helps to inhibit RNF168 recruitment at telomeres. Function This protein is present at telomeres in metaphase of the cell cycle, is a second negative regulator of telomere length, and plays a key role in the protective activity of telomeres. While having similar telomere binding activity and domain organization, TERF2 differs from TERF1 in that its N terminus is basic rather than acidic. T-Loop formation Telomeric ends are structurally similar to double-stranded breaks on the chromosome. To prevent the cellular DNA repair machinery from mistakenly identifying telomeres as chromosome breaks, t-loops are formed in which the 3’ TTAGGG overhang of the telomere loops back into the DNA duplex. TERF2 promotes t-loop formation by preferentially binding to a telomeric double-stranded DNA duplex containing a 3’ TTAGGG single-stranded overhang. If the 3’ TTAGGG overhang is not present, TERF2 will not bind. Once bound, it migrates to the t-loop junction where the single-stranded overhang invades the double-stranded region upstream. No other shelterin protein has been shown to promote this process but interaction of TERF2 with TERF2IP is shown to promote higher t-loop formation in vitro. Studies have demonstrated that deletion of TERF2 prevents t-loop formation, leading to excessive loss of telomeric DNA and early cell death. ATM kinase prevention TERF2 plays a central role in preventing ATM kinase DNA damage response. It binds telomeric dsDNA and prevents telomeres from activating ATM kinase. This interaction of TERF2 with ATM is believed to be relevant to the mechanism by which TERF2 blocks ATM signaling. Because of its oligomeric nature, TERF2 could potentially cross-link ATM monomers and hold the kinase in its inactive dimeric state, thereby blocking amplification of the ATM signal at an early step in its activation. However, because mutations in the TERF2 dimerization domain destabilize the protein, it has not been possible to test