Passaging on selective media of yeast strains transformed with complete 2 micron vectors carrying TRP1, LEU2 or URA3 selective markers leads to curing of the resident endogenous 2 micron DNA in a majority of the population. Vector plasmids defective in FLP function are fixed as populations of A, A + B or B forms after 2 micron loss. Transformation with these plasmids offers a general method of obtaining cir degree derivatives of any yeast strain.