Loss of 2 um DNA from Saccharomyces cerevisiae transformed with the chimaeric plasmid pJDB219

Summary Two plasmids containing Saccharomyces cerevisiae 2 µm DNA sequences and the S. cerevisiae LEU2 gene have been found to display incompatibility with 2 µm DNA; in the presence of the LEU2 plasmids, 2 µm DNA can be lost. The LEU2 plasmids can be lost spontaneously after (and before) 2 µm DNA loss has occurred, so that strains completely lacking 2 µm DNA sequences can be obtained routinely.