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Fragment spanning the SV40 replication origin is the only DNA sequence required in cis for viral excision.

A 311-base pair fragment containing the SV40 origin of replication was linked to the chicken thymidine kinase gene on a recombinant plasmid. This molecule was transfected into human 143 thymidine kinase-deficient (TK-) cells, and colonies positive for thymidine kinase were selected. When cell lines derived from these colonies were fused to permissive simian cells that produce SV40 T antigen, the recombinant plasmid excised itself from the human cellular genome and replicated with a high copy number per cell. These results show that this segment of the viral genome is the only sequence required in cis to mediate SV40 excision and replication upon fusion to permissive cells. In addition, we have shown that excised plasmids apparently identical to the input DNA can be efficiently rescued in Escherichia coli. SV40 excision and replication may therefore be useful for the recovery of cloned genes from eukaryotic cells.