A DNA fragment called suc1 has been found to rescue cells mutated in the cell cycle control gene cdc2 of the fission yeast Schizosaccharomyces pombe. The suppressing activity of suc1 is observed when it is present on a multicopy number plasmid. The gene does not hybridize to cdc2 and maps elsewhere in the genome. Its effect is cdc2 allele specific suggesting that it interacts directly with the cdc2 gene function.