The invention provides compositions and methods for gene therapy using cytolethal distending toxins (CDTs). In a preferred embodiment, a gene therapy vector according to the invention includes a gene encoding a B subunit of a CDT and an antisense oligonucleotide that inhibits a DNA repair mechanism. An inducible promoter is operably linked to the gene and oligonucleotide. Preferably, the promoter is strictly inducible by heat shock.