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.

Abstract: A method for creating an alphavirus-based genomic library, comprising a) ligation of foreign sequence (s) from an expression library or a random library into plasmids containing cloned alphaviral cDNA, b) multiplication of the obtained plasmid constructs in bacterial cells, c) direct transfection of the obtained plasmid constructs into mammalian or arthropod cells, characterized in that the sequence of an intron or sequences of introns are inserted into the respective genome of an alphavirus or into the cDNA of an expression vector based on an alphavirus, —the sequence of a viral subgenomic promoter, which is larger than minimal functional promoter is inserted immediately to the 3? end of the sequences coding the structural proteins of the named alphavirus, —and ribozyme sequence is inserted for creating correct 3? ends of the alphavirus.