Delta-integration vectors were constructed for the purpose of achieving homologous integration of the hirudin expression cassette into the chromosome of Saccharomyces cerevisiae. A double $delta$ system truncated with the unnecessary bacterial genes, and consequently having a reduced insert size for integration, showed a four-fold increase in transformation efficiency at given DNA concentrations, and as a result, the constructed recombinant yeast strain had a 1.3-fold enhancement in hirudin expression level compared with a single $delta$ system.