DNA polymerases mu (POLM) and lambda (POLL) facilitate non-homologous end joining (NHEJ) of DNA double strand breaks (DSBs) by filling single strand (ss) gaps (usually 1- or 2- nucleotide gaps) present at DNA DSB ends positioned for ligation in the synaptic complex containing XRCC5:XRCC6 (Ku), PRKDC (DNA-PKcs), DCLRE1C (ARTEMIS), XRCC4:LIG4 and NHEJ1 (XLF) (Mahajan et al. 2002, Lee et al. 2004, Fan and Wu 2004, McElhinny et al. 2005, Davis et al. 2008).