Telomeric ends are protected by shelterin complexes, which are composed of six proteins, including TRF1, TRF2, RAP1, POT1, TIN2, and TPP1 (Figure 5B), and their lengths are maintained by telomerase activity regulated by the complexes, depending on cell type (cancer cells and normal cells) [20,21]. The gene discussed is POT1; the disease is cancer.