The Saccharomyces cerevisiae cdc9 gene has been cloned in the vector YRp12 by complementation of the temperature-sensitive lesion in vivo. The gene is contained within a 3300-base-pair fragment of DNA, which maps to the chromosomal locus of cdc9 and which is able to complement a DNA-ligase-deficient mutant of the fission yeast Schizosaccharomyces pombe.