The development of random genomic DNA libraries was an important technological breakthrough that enabled the physical isolation of any gene. Random libraries continue to be used in genetic screens and for cloning genes, but they suffer from the problems that not all portions of the genome are present, and that not all portions are present at equal levels. The goal of this project is to overcome these problems by constructing systematic libraries of the Saccharomyces cerevisiae genome. Significant progress has been made towards this goal, but physical and functional gaps remain. These gaps will be filled, and the inserts from the entire library will be transferred to another vector, creating a matching pair of systematic libraries for low level expression and for overexpression of every yeast gene. The resulting libraries will enable efficient and comprehensive overexpression screens and facilitate cloning of genes by plasmid complementation.