Abstract: Viral vectors are potential tools for eliminating the viability of eukaryotic cells in anti-cancer therapies since they can efficiently destroy the cancer cells and trigger an immune response against tumours. Typically viruses are not specific to cancer cells and all methods known in art aiming to the construction of cancer-specific viruses suffer from serious problems. The present invention presents a universal method to overcome these problems and is usable for any DNA virus replicating in nucleus or for any layered vector of RNA viruses. In this method the viral gene expression and/or replication will be blocked by the introduction of one or more aberrantly spliced introns into crucial gene expression units of the virus or vector. Lethal effect of these mutations is reverted in a controlled manner by the delivery of splice-switch oligonucleotide (s) correcting the introduced defects and restoring the biological functionality of the virus or vector, including cytolytic properties.