Cell-cycle-specific regulator of classical non-homologous end joining (NHEJ) of DNA double-strand break (DSB) repair, which can act both as an activator or inhibitor of NHEJ, depending on the cell cycle phase (PubMed:24610814, PubMed:28959974). Acts as a regulator of DNA repair pathway choice by specifically inhibiting classical NHEJ during the S and G2 phases, thereby promoting error-free repair by homologous recombination during cell cycle phases when sister chromatids are present (PubMed:28959974). Preferentially protects single-stranded overhangs at break sites by inhibiting classical NHEJ, thereby creating a local environment that favors homologous recombination (PubMed:28959974). Acts via interaction with XRCC5/Ku80 and XRCC6/Ku70 (PubMed:28959974). In contrast, acts as an activator of NHEJ during G1 phase of the cell cycle: promotes classical NHEJ in G1 phase cells via multivalent interactions that increase the affinity of DNA damage response proteins for DSB-associated chromatin. Also involved in immunoglobulin V(D)J recombination (By similarity). May also act as an indirect regulator of proteasome (By similarity).