Contributes to 3' overhang single-stranded DNA endodeoxyribonuclease activity. Involved in double-strand break repair via nonhomologous end joining; negative regulation of protection from non-homologous end joining at telomere; and telomeric DNA-containing double minutes formation. Acts upstream of or within several processes, including DNA metabolic process; hemopoiesis; and nervous system development. Located in chromosome, telomeric region. Part of DNA-dependent protein kinase-DNA ligase 4 complex and ERCC4-ERCC1 complex. Is expressed in head; heart; and trunk. Human ortholog(s) of this gene implicated in multiple myeloma. Orthologous to human XRCC4 (X-ray repair cross complementing 4). [provided by Alliance of Genome Resources, Apr 2022]