Component of the spindle-assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at the metaphase plate. May recruit MAD2L1 to unattached kinetochores. Has a role in the correct positioning of the septum. Required for anchoring MAD2L1 to the nuclear periphery. Binds to the TERT promoter and represses telomerase expression; possibly by interfering with MYC binding. {ECO:0000269|PubMed:10049595; ECO:0000269|PubMed:12837246; ECO:0000269|PubMed:20133940}.