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A selective scheme is presented for isolating sequences capable of replicating autonomously in the yeast Saccharomyces cerevisiae. YIp5, a vector that contains the yeast gene ura3, does not transform a ura3 deletion mutant to Ura+. Hybrid YIp5-Escherichia coli DNA molecules also fail to produce transformants. However, collections of molecular hybrids between YIp5 and DNA from any of six eukaryotes tested (S. cerevisiae, Neurospora crassa, Dictyostelium discoideum Ceanhorabditis elegans, Drosophila melanogaster, and Zea mays) do transform the deletion mutant. The Ura+ transformants grow slowly, are unstable under nonselective conditions, and carry the transforming DNA as autonomously replicating, supercoiled circular molecules. Such a phenotype is qualitatively identical to that of strains transformed by molecules containing a yeast chromosomal origin of replication. Thus, these DNA hybrid …