these organisms survive DSBs induced during desiccation. It preferentially use rNTPs (RNA nucleotides), possibly advantageous in dormant cells. The archaeal NHEJ system in Methanocella paludicola have a homodimeric Ku, but the three functions of LigD are broken up into three single-domain proteins sharing an operon. All three genes retain substantial homology with their LigD counterparts and the polymerase retains the preference for rNTP. NHEJ has been lost and acquired multiple times in bacteria and archaea, with a significant amount of horizontal gene transfer shuffling the system around taxa. Corndog and Omega, two related mycobacteriophages of Mycobacterium smegmatis, also encode Ku homologs and exploit the NHEJ pathway to recircularize their genomes during infection. Unlike homologous recombination, which has been studied extensively in bacteria, NHEJ was originally discovered in eukaryotes and was only identified in prokaryotes in the past decade. In eukaryotes In contrast to bacteria, NHEJ in eukaryotes utilizes a number of proteins, which participate in the following steps: End binding and tethering In yeast, the Mre11-Rad50-Xrs2 (MRX) complex is recruited to DSBs early and is thought to promote bridging of the DNA ends. The corresponding mammalian complex of Mre11-Rad50-Nbs1 (MRN) is also involved in NHEJ, but it may function at multiple steps in the pathway beyond simply holding the ends in proximity. DNA-PKcs is also thought to participate in end bridging during mammalian NHEJ. Eukaryotic Ku is a heterodimer consisting of Ku70 and Ku80, and forms a complex with DNA-PKcs, which is present in mammals but absent in yeast. Ku is a basket-shaped molecule that slides onto the DNA end and translocates inward. Ku may function as a docking site for other NHEJ proteins, and is known to interact with the DNA ligase IV complex and XLF. End processing End processing involves removal of damaged or mismatched nucleotides by nucleases and resynthesis by DNA polymerases. This step is not necessary if the ends are already compatible and have 3' hydroxyl and 5' phosphate termini. Little is known about the function of nucleases in NHEJ. Artemis is required for opening the hairpins that are formed on DNA ends during V(D)J recombination, a specific type of NHEJ, and may also participate in end trimming during general NHEJ. Mre11 has nuclease activity, but it seems to be involved in homologous recombination, not NHEJ. The X family DNA polymerases Pol λ and Pol μ (Pol4 in yeast) fill gaps during NHEJ. Yeast lacking Pol4