A selection procedure was devised to study the role of cis -acting sequences at origins of DNA replication. Two regions in Herpes simplex virus oriS were examined: an AT-rich spacer sequence and a putative binding site, box III, for the origin binding protein. Plasmid libraries were generated using oligonucleotides with locally random sequences. The library, amplified in Escherichia coli , was used to transfect BHK cells followed by superinfection with HSV-1. Replicated plasmids resistant to Dpn I cleavage were amplified in E. coli. The selection scheme was repeated. Plasmids were isolated at different stages of the procedure and their replication efficiency was determined. Efficiently replicating plasmids had a high AT content in the spacer sequence as well as a low helical stability of this region. In contrast, this was not seen using the box III library. We also noted that the wild type sequence invariably dominated the library after five rounds of selection. These plasmids arose from recombination between plasmids and viral DNA. Our results imply that a large group of sequences can mechanistically serve as origins of DNA replication. In a viral system, however, where the initiation process might be rate-limiting, this potentially large group of sequences would always converge towards the most efficient replicator.