A set of four yeast shuttle vectors that incorporate sequences from the Saccharomyces cerevisiae 2 mu endogenous plasmid has been constructed. These yeast episomal plasmid (YEp)-type vectors (pRS420 series) differ only in their yeast selectable markers, HIS3, TRP1, LEU2 or URA3. The pRS420 plasmids are based on the backbone of a multifunctional phagemid, pBluescript II SK+, and share its useful properties for growth in Escherichia coli and manipulation in vitro. The pRS420 plasmids have a copy number of about 20 per cell, equivalent to that of YEp24. During non-selective yeast growth, pRS420 plasmids are lost through mitotic segregation at rates similar to other YEp vectors and yeast centromeric plasmid (YCp) vectors, in the range of 1.5-5% of progeny per doubling. The pRS420 series provides high-copy-number counterparts to the current pRS vectors [Sikorski and Hieter, Genetics 122 (1989) 19-27].