Retroviral DNA integration is catalyzed by the viral protein integrase. Here, it is shown that DNA-dependent protein kinase (DNA-PK), a host cell protein, also participates in the reaction. DNA-PK-deficient murine scid cells infected with three different retroviruses showed a substantial reduction in retroviral DNA integration and died by apoptosis. Scid cell killing was not observed after infection with an integrase-defective virus. suggesting that abortive integration is the trigger for death in these DNA repair-deficient cells. These results suggest that the initial events in retroviral integration are detected as DNA damage by the host cell and that completion of;the integration process requires the DNA-PK-mediated repair pathway.