In budding yeast, the cell cycle is negatively regulated at G1 during vegetative growth. Daughter cells born smaller than a critical cell size have to accumulate mass before they bud. In order to identify genes involved in the negative regulation of the cell cycle in response to cell size, a genetic screen was carried out to isolate dsl ( CLN3-2/ DAF1-1 synthetic lethal) mutants. CLN3-2 is a dominant, hyperstable/hyperactive allele of the G1 cyclin gene CLN3. CLN3-2 cells are smaller and have a very short G1 phase. Interestingly, the CLN3-2 strain has the same doubling time as wild type strains. It appears that a shortened G1 phase is compensated by lengthening of the G2 phase. dsl mutants might thus be defective in this compensation mechanism. Two mutants have been isolated thus far in the pilot screen, which are inviable when CLN3-2 is expressed from GAL1, 10 promoter. The dsl1 mutant is also temperature sensitive and the dsl2 mutant has an irregular colony morphology phenotype. We are currently cloning both DSL1 and DSL2.