Fig. 3

Alternative non-homologues end joining pathway. A PARP1 binds to single-strand DNA for the recognition of damages. It catalyzes the poly-ADP-ribosylation of proteins at DNA damage sites. Also, it contributes to the initial assembling of the MRN complex on DSBs, leading to the activation of ATM and ATR kinases. B In SSA, the combination of MRN and CtIP creates short ssDNA, and then, EXO1 or BLM/DNA2 endonuclease complex causes an extensive end resection. EXO1 is loaded on ssDNA by Metnase. C RAD52 binds to the RPA-coated single strand annealing the complementary regions. ERCC1/XPF complex removes the tails. This step of DNA end resection is dispensable for MMEJ. D Then, polymerases, flap endonucleases, helicases, and polynucleotide kinases prepare the DNA ends for ligation (the gap-filling component of SSA is unidentified). E Finally, Lig III ligates the DNA ends with the help of XRCC1 as a scaffolding protein