Vens, C., Hofland, I. and Begg, A. C. Involvement of DNA Polymerase Beta in Repair of Ionizing Radiation Damage as Measured by In Vitro Plasmid Assays. Radiat. Res. 168, 281– 291 (2007). Characteristic of damage introduced in DNA by ionizing radiation is the induction of a wide range of lesions. Single-strand breaks (SSBs) and base damages outnumber double-strand breaks (DSBs). If unrepaired, these lesions can lead to DSBs and increased mutagenesis. XRCC1 and DNA polymerase beta (polβ) are thought to be critical elements in the repair of these SSBs and base damages. XRCC1-deficient cells display a radiosensitive phenotype, while proliferating polβ-deficient cells are not more radiosensitive. We have recently shown that cells deficient in polβ display increased radiosensitivity when confluent. In addition, cells expressing a dominant negative to polβ have been found to be radiosensitized. Here we show that repair of radiation-induced lesions is inhibited in extracts with altered polβ or XRCC1 status, as measured by an in vitro repair assay employing irradiated plasmid DNA. Extracts from XRCC1-deficient cells showed a dramatically reduced capacity to repair ionizing radiation-induced DNA damage. Extracts deficient in polβ or containing a dominant negative to polβ also showed reduced repair of radiation-induced SSBs. Irradiated repaired plasmid DNA showed increased incorporation of radioactive nucleotides, indicating use of an alternative long-patch repair pathway. These data show a deficiency in repair of ionizing radiation damage in extracts from cells deficient or altered in polβ activity, implying that increased radiosensitivity resulted from radiation damage repair deficiencies.