5-Fluoroorotic acid (otherwise referred to as '5-FOA') is used to counterselect URA3 expressing yeast. The URA3 gene product (orotine-5'-monophosphate decarboxylase) converts 5-FOA to 5-fluorouracil, which is toxic.
Working concentration of 5-FOA is 1mg/mL. It can be stored as a concentrated stock solution in DMSO (100 mg/mL) or as a powder. 5-FOA is sparingly soluble in water, and slightly soluble (<50 mg/mL) in 1:1 NH4OH*H2O.
5-FOA is not particularly toxic, but the MSDS recommends working in a hood to avoid inhalation. DMSO solutions pose additional (DMSO-related) risks.
Store at -20.