E1B-55kD-deleted adenoviruses have been used as conditionally replicative adenoviruses (CRAds) for therapeutic purposes in tumors with loss-of-function p53 mutation. To target cancer cells that harbor activating mutant KRAS (KRAS aMut ) but spare p53 wild normal cells, we constructed and examined by reporter assays a KRAS aMut but not p53-responsive promoter, the Δp53REP2 promoter. The Δp53REP2 promoter, derived from human double minute 2 (hdm2) P2 promoter with its p53 response elements being deleted, was used to regulate the expression of the hdm2 transgene in a novel E1B-55kD-deleted CRAd, the