Telomere-binding protein that acts as a regulator of telomere length. Directly binds the telomeric double-stranded 5'-TTAGGG-3' repeat. Preferentially binds to telomeres that have a low concentration of shelterin complex and acts as a regulator of telomere length by initiating telomere trimming, a process that prevents the accumulation of aberrantly long telomeres. Also acts as a transcription regulator that binds to promoter regions. Regulates expression of a small subset of genes, including MTFP1. Regulates expression the J and/or S elements in MHC II promoter. Acts as a negative regulator of cell proliferation by specifically activating expression of ARF, a tumor suppressor isoform of CDKN2A.