**1. Introduction**

596 DNA Repair

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Most eukaryots have specialized protein-DNA complexs, called telomeres at the ends of natural linear chromosomes. Telomeric DNA consists of a tandemly repeated G-rich sequence. The lengths of telomeric DNAs in *S. pombe*, *S. cerevisiae,* and human are ~300 nucleotids, ~350 nucleotides, and ~10 kb, respectively. The ends of the telomeric DNA have 3' single-stranded overhangs. The protein components of telomeres consists of doublestranded telomere-binding proteins, such as human TRF1 and TRF2, S. pombe Taz1, and single-stranded telomere-binding proteins, such as *S. cerevisiae* Cdc13, *S. pombe* Pot1, and human POT1. DNA double-strand breaks (DSBs) must be repaired to maintain genomic integrity. In contrast, natural chromosome ends should not be recognized as DSBs. The telomere is capped to protect from DNA repair activity. If this capping function is lost, this uncapped telomere is recognized as DNA damage and becomes substrate for DNA repair proteins. The first step in homologous recombination (HR) repair is processing of DNA ends by 5' to 3' degradation to create 3' single-stranded overhangs. The proteins involved in this steps include *S. cerevisiae* Mre11-Rad50-Xrs2 complex (MRX), Sae2, Sgs1, and Dna2. Recent works revealed that proteins involved in the processing of DNA DSB ends are also involved in the processing of capped or uncapped telomere. These facts raised new question of how these proteins are regulated at telomere ends. This chapter will focus on the roles of proteins involved in the processing of DBS ends at capped (functional) and uncapped (dysfunctional) telomere in *S. pombe*, *S. cerevisiae* and human. This chapter will also focus on the functional interactions between telomere-binding proteins and proteins involved in the processing of DBS ends. Resent works revealed that double-stranded and single-stranded telomerebinding proteins play critical roles to control proteins involved in DNA repair at chromosome ends.
