DNA polymerase mu shares a number of characteristics with DNA polymerase Beta as well as with terminal deoxynucleotideyltransferase. Pol mu purportedly plays a role in microhomology mediated joining and the repair of double-stranded breaks. However, unlike other DNA polymerases, which show substrate specificity for deoxynucleotides, DNA Pol mu incorporates both deoxynucleotides and ribonucleotides in a template- directed manner. This unusual capability implies a novel role for this polymerase in DNA repair.