Involved in non-homologous end joining (NHEJ). Facilitates the transport of LIF1 into the nucleus, where it can interact with DNA ligase DNL4 to repair double-strand breaks (DSB). Mediates mating-type regulation of NHEJ. Prevents chromosome circularisation by NHEJ in absence of telomerase. Repressed in diploid cells. Present with 377 molecules/cell in log phase SD medium. Belongs to the XRCC4-XLF family. XLF subfamily.