Telomeric repeat binding factor 1 (TERF1, PIN2, TRF1, TRBF1) and 2 (TERF2, TRF2, TRBF2) are present at telomeres throughout the cell cycle where they regulate telomerase by acting in cis to limit the elongation of individual chromosome ends. Telomerase adds hexameric repeats of 5'-TTAGGG-3' to the ends of chromosomal DNA. This telomerase enzyme plays an influential role in cellular immortalization and cellular senescence. TRF1 negatively regulates telomere elongation, while TRF2 protects the chromosome ends by inhibiting end-to-end fusions. Down-regulation of TRF expression in tumor cells may contribute to cell immortalization and malignant progression. TRF1 has an acidic N-terminus while TRF2 has a basic N-terminus.
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