Abstract: This invention relates to the fields of genetic engineering, virus replication and gene transfer. More specifically, this invention relates to polynucleotide construct, recombinant virus, transposon, and their vectors, wherein an ori derived from a DNA virus capable of replicating in vertebrate cells is inserted into the retrovirus, allowing the retrovirus following the reverse transcription to efficiently replicate as extrachromosomal or episomal DNA without the necessity of integration into the host cell chromosome. Additionally, this invention relates to polynucleotide construct, recombinant virus, transposon, and their vectors replicating episomally without aid of an ori and related elements. Also, this invention encompasses preventive, therapeutic, and diagnostic applications employing said constructs, viruses and vectors.