Telomeric repeat binding factor 1 (TRF1, TERF1, PIN2, TRBF1) and telomeric
repeat binding factor 2 (TRF2, TERF2, 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. Downregulation 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. TRF2 localizes in the nucleolus at G0 and S and diffuses out of
the nucleolus in G2 phase. During mitosis TRF2 disperses from the condensed
chromosomes and returns to the nucleolus at cytokinesis.