Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, genome instability, and radiation sensitivity. Previous research has shown that it is possible to correct the hereditary deficiency A-T by DNA transfection in cell culture, but the large size of the ATM cDNA (9 kb) limits the use of many vector types for gene replacement. HSV-1 amplicon vectors provide a means to deliver large genes to cells efficiently and without toxicity. In this study, the FLAG-tagged cDNA for human ATM was inserted into an HSV-1 amplicon under control of the CMV promoter (designated as HGC-ATM). FLAG-ATM expression was confirmed in 293T/17 cells and human A-T fibroblasts (GM9607) after transduction, by immunoprecipitation, Western analysis, and immunocytochemistry. Functional recovery was assessed by two independent assays. First, in vitro kinase assay showed that vector-derived ATM in GM9607 cells could successfully phosphorylate wt p53 using recombinant GST-p53(1-101). Second, in A-T cells infected with the HGC-ATM vector, the extent of accumulation in G2/M phase at 24 h postirradiation was similar to that observed in cells with wild-type endogenous ATM and lower than that observed in A-T cells infected with a control vector. Thus, these vectors provide a tool to test the feasibility of HSV-amplicons as gene therapy vectors for A-T.