Figure 3.

Joining by tolerance. The NHEJ core machinery can be associated with an XLF-XRCC4 filament that may stabilize aligned complex end structures (i) sufficiently to support transfer of the adenylate from the ligase to 5^′ phosphate terminus (ii), possibly through to complete ligation (iii). If ligation aborts, the 5^′ adenylated (AMP) intermediate can be removed by Aprataxin (APTX) (iv). DNA-PKcs-directed phosphorylation of XLF and XRCC4 can disrupt the XLF-XRCC4 filament (v) to allow processing factors access to the DNA to remove the obstruction.