The budding yeast Saccharomyces cerevisiae
undergoes a cell cycle similar to other eukaryotic organisms except for the lack of nuclear envelope dissolution during mitosis and the production of daughter cells via
budding, and thus budding yeast has become a model system for studying eukaryotic cell cycle progression 
due to its rapid division, the availability of genetic tools, and homology to higher eukaryotic cell cycle processes. Numerous genes and proteins are involved in directing cells through the 4 major cell cycle phases, the growth gap phase G1, the DNA synthesis (S) phase, a second growth gap phase G2, and the mitotic (M) cell division phase 
. Extensive effort has been made to decipher the mechanisms of cell cycle control. However, given the extreme complexity of the cell cycle, with ~300–800 genes regulated in a cell cycle-dependent manner 
, the complete set of cell cycle regulators, effectors, and helper proteins has yet to be determined.
Classically, conditional temperature-sensitive mutants have been very effective for studying yeast cell cycle division. Hartwell and colleagues identified more than 50 cell division cycle (CDC) genes required at specific stages in cell cycle division, by identifying conditional temperature-sensitive mutants with specific arrest points 
. Gene dosage has been another powerful approach to study gene function. Either increasing (overexpression) or decreasing gene dosage (gene deletion or gene knockdown) can influence the activity of genes and lead to detectable phenotypes. Most large-scale cell cycle screens have focused on studying cell cycle progression by employing loss-of-function approaches such as gene deletion, RNAi, and promoter shutoff 
and have successfully identified many cell cycle genes. However, loss-of-function mutations can often be masked, such as in the cases of genes acting as negative regulators or genes compensated for by redundant functions 
. In contrast, overexpression of a gene product can potentially overcome such effects and often leads to a more detectable effect on cellular function 
. Overexpression also offers the opportunity to identify and study gain-of-function mutations.
In order to identify additional cell cycle genes, especially those difficult to identify in loss-of-function studies, large-scale screens focusing on the effects of overexpression-induced gain-of-function of genes in cell-cycle progression are needed. Stevenson et al.
performed the first such large-scale overexpression screen for cell cycle genes by expressing a moderated GAL promoter-driven cDNA library and sheared genomic DNA pool in ARS-CEN vectors 
. Although 113 genes, including those causing only slight effects on the cell cycle, were identified from this screen, this screen was unsaturated due to the coverage of the cDNA library and incomplete gene annotation. Therefore, completion of the S. cerevisiae
genome sequence and the systematic cloning of all genes into overexpression vectors now allow a more comprehensive analysis of the set of genes.
Analysis of overexpression phenotypes using cell sorting to assay the distribution of cells in different cell cycle stages has the advantage of being more quantitative and discerning than simple growth screens. However, flow cytometry has not been carried out comprehensively to cover all genes in the genome. In the present work, we performed a near-saturating screen for yeast genes having overexpression-induced defects in cell cycle progression, taking advantage of the availability of a yeast open reading frame (ORF) clone collection covering 91% of the yeast complete ORF set, including dubious ORFs 
. After measuring the fraction of cells in different phases of the cell cycle via
high-throughput flow cytometry for each of 5,556 individual ORFs and performing secondary validation assays, we identified 108 genes whose overexpression leads to significant changes in the timing of passage through the G1 or G2/M stages of the cell cycle. 82 of these genes are newly implicated in the cell cycle, with the majority likely to affect cell cycle progression via