APEX1 and xeroderma pigmentosum group A: SIRT1, a NAD+-dependent deacetylase, has been demonstrated to directly interact with and deacetylate multiple core DNA repair factors, including Ku70 (15) (critical for non-homologous end joining), apurinic/apyrimidinic endonuclease-1 (APE1) (base excision repair effector) (75), NBS1 (nibrin/p95, a central component of the MRE11-RAD50-NBS1 [MRN] complex for DNA damage sensing) (76), and xeroderma pigmentosum group A (XPA) (key mediator of nucleotide excision repair) (77).