Recycling selectable markers in yeast.

A series of excisable marker cassettes has been constructed to facilitate recycling of selectable markers in yeast. These cassettes exploit the use of the Cre DNA recombinase to precisely excise the marker gene when desired. They are especially useful for making gene disruptions and then removing the marker gene to allow subsequent genetic manipulations with that same marker. Also described are a number of cre expression vectors that allow galactose-induced expression of the recombinase in yeast. The procedure is simple and allows rapid processing of large numbers of transformants.