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Molecular genetic tools have become increasingly important in fundamental and applied yeast research. Genetic modifications, such as the targeted inactivation of genes or their controlled (over) expression from episomal or integrating vectors, require the use of selectable marker genes for efficient detection and selection of transformed cells (4). After genetic modification, marker genes may be applied to counterselect against revertants. Especially during gene expression from episomal vectors, this counterselection is often a prerequisite for avoiding overgrowth of the population with cells that have lost the expression vector (18, 33).

Several marker genes used in yeast genetics confer resistance against antibiotics or other toxic compounds (42). Selection for strains that carry such marker genes requires the addition of these toxic compounds to the growth media. In addition to their toxicity, the price of many of these …