The authors found that in breast cancer and osteosarcoma cell lines that maintain their telomere length through ALT, the telomere-associated protein SLX4IP promotes RAP1 SUMOylation by PIAS1, driving the detachment of RAP1 from telomeres and shuttling from the nucleus to the cytoplasm, which enhances its extratelomeric function. Here, TERF2IP is linked to osteosarcoma.