Using 5-Fluoroorotic Acid (5-FOA) for the counter-selection of yeast is a common genetic screening method. Curing yeast strains of plasmids, plasmid shuffling, allelic replacement, and two-hybrid screens are methods that can employ the use of 5-FOA. Otherwise nontoxic to yeast, 5-FOA is converted to the toxic form (i.e., 5-flurouracil) in strains expressing the functional URA3 gene coding for orotine-5'-monophosphate decarboxylase that is involved in the synthesis of uracil. Yeast strains that are phenotypically Ura+ become Ura- and 5-FOA(R) (resistant) after selection.<br>The question of 5-FOA solubility is often raised by customers using ultra-pure (< 98%) 5-FOA powder because of its insolubility in water. Thus, we provide a 100X concentrated (100 mg/ml) 5-FOA solution in DMSO and a 2X concentrated (2 mg/ml) synthetic complete 5-FOA (i.e., 2X SC/5-FOA) in liquid medium containing yeast nitrogen base, ammonium sulfate, complete amino acids, and glucose. Both have been tested and validated on the basis of counter selection activity.