Abstract: In order to characterize new yeast genes regulating cell proliferation, a number of overexpression-sensitive clones have been isolated from a Saccharomyces cerevisiae cDNA library in a multicopy vector under the control of the GAL1 promoter, on the basis of growth arrest phenotype under galactose-induction conditions. Thirteen of the independent clones isolated in this way correspond to previously known genes (predominantly coding for morphogenesis-related proteins or for multifunctional transcriptional factors), while the remaining 11 independent clones represent new genes with unknown functions. The more stringent conditions employed in this screening compared with previous ones that also employed a dominant genetics approach to isolate overexpression-sensitive genes has allowed us to extend the number of yeast genes that exhibit this phenotype. The effect of overexpression of MCM1 (whose product participates in the regulation of a number of apparently unrelated cellular functions) has been studied in more detail. Galactose-induced overexpression of MCM1 leads to rapid growth arrest at the G1 or S cell cycle stages, with many morphologically-abnormal cells. Several of the other clones also exhibit a G1 arrest terminal phenotype when overexpressed.