A conditionally replicating adenovirus were generated that can specifically replicate and express therapeutic genes in neuroendocrine tumors. The promoter-specific expression of the adenoviruses is regulated upstream by an INSM1 (insulinoma-associated-1) promoter that is silent in normal adult tissues but active in developing neuroendocrine cells and neuroendocrine tumors. By placing the I NSM 1-promoter with an insulator and two copies of neuronal restrictive silencer elements in an adenoviral vector, the construct can retain tumor specificity and drive expression of a mutated adenovirus E1A gene (Δ24E1A) and the herpes simplex virus thymidine kinase gene. The I NSM1-promoter-driven viruses could replicate specifically in the I NSM1-positive cells and I NSM1-specific HSV-tk expression in combination with ganciclovir treatment displayed dose-dependent tumor cell-specific killing in insulinomas. When the INSM1-promoter driven HSV-tk was combined with Δ24E1A and I NSM 1 p-HSV-tk viruses, the co-infected insulinoma expressed higher levels of HSV-tk and more efficient tumor suppression as compared to the I NSM1 p-HSV-tk virus alone.