N-terminal head domain found in the XRCC4 superfamily of proteinsThe XRCC4 superfamily includes five families: XRCC4, XLF, PAXX, SAS6 and CCDC61. XRCC4 (X-ray repair cross-complementing protein 4), XLF (XRCC4-like factor) and PAXX (paralog of XRCC4 and XLF) play crucial roles in the non-homologous end-joining (NHEJ) DNA repair pathway. SAS6 (spindle assembly abnormal protein 6) and CCDC61 (coiled-coil domain-containing protein 61) have a centrosomal/centriolar function. Members of this superfamily have an N-terminal globular head domain, a centrally located coiled-coil, and a C-terminal low-complexity region. They form homodimers through two homodimerization domains: an N-terminal globular head domain and a parallel coiled-coil domain. In addition, some members such as XRCC4 and XLF form symmetric heterodimers that interact through their globular head domains at the opposite end of the homodimer interface, and may form XLF-XRCC4 filaments. This model corresponds to the N-terminal head domain of XRCC4 superfamily proteins.