XRCC4 plays a crucial role in the DNA repair process, specifically in the non-homologous end joining pathway, which is essential for repairing double-strand breaks in DNA. This function is vital for maintaining genomic stability, as unrepaired DNA breaks can lead to mutations, cancer, and cell death. XRCC4 acts as an accessory factor for DNA ligase IV, enhancing its ability to join DNA fragments with nicks or broken ends, thereby facilitating the repair process. Additionally, XRCC4 is involved in V(D)J recombination, a critical mechanism for generating diversity in immune responses. Deficiencies in XRCC4 or its interacting partners can result in severe immunodeficiency and increased susceptibility to cancer, highlighting the importance of this protein in both DNA repair and immune system functionality.