Analysis of the genome of Saccharomyces cerevisiae (brewer's and baker's yeast) has provided an opportunity for developing new tools for the functional analysis of eukaryotic genomes. One of the most powerful approaches is the generation of specific deletion mutants. Phenotypic analysis of the mutant strain may well reveal the function of the missing protein. To date, classic processes for systematic disruption require several steps: cloning of the gene into a plasmid, construction of a restriction map, and replacement or elimination of the coding region of the gene. The procedure is time-consuming and has not been carried out in a systematic way.