One essential step for the molecular dissection of gene function is gene inactivation. In the yeast Saccharomyces cerevisiae, elaborate tools for gene disruption are available. Gene disruption cassettes carrying completely heterologous marker genes flanked by short DNA segments homologous to the corresponding regions left and right of the gene to be deleted result in highly efficient one-step gene disruption events yielding usually more than 50% of the clones carrying the correctly disrupted gene. Presence of loxP sites flanking the disruption marker gene allows Cre recombinase-mediated marker rescue so that the marker can be used to disrupt another gene.