Negatively regulates telomere length by preventing telomere elongation or promoting degradation of the telomere ends. Recruited to telomeres by interaction with the C-terminus of RAP1, which binds directly to telomeric repeat DNA. This may create a negative feedback loop in which the addition of new telomere repeats creates binding sites for inhibitors of telomere length extension. May also influence the balance of transcriptional silencing at telomeres and the silent mating type locus HMR, which is mediated by SIR (Silent Information Regulator) proteins including SIR3 and SIR4. RIF1 competes with SIR proteins for binding to the C-terminus of RAP1. In the absence of RIF1, a limiting cellular pool of SIR proteins may preferentially associate with RAP1 at sub-telomeric loci, causing enhanced telomeric silencing and attenuated silencing of the HMR locus. Interacts with RIF2 and RAP1. Localizes to telomeres. Telomere association begins in G1/S, attains a peak during late G2/S phase of the cell cycle, and is lost during telophase. Localization to telomeres may be increased by telomere uncapping caused by expression of a mutant telomerase RNA subunit (TLC1). Belongs to the RIF1 family.