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5-FOA is used to select for URA negative cells. URA positive cells convert 5-FOA to 5-fluorouracil, which is toxic. It's generally used when you want to rescue a plasmid or integration that used URA as the selection marker. That allows you to reuse the URA marker later.


  • Dissolve 5-FOA (from the door of the main -20C freezer) in DMSO (from the back hood) at 100 mg/mL.
  • This is a 100x stock. Add to plates or media as you would antibiotics (i.e. let the media cool before adding 5-FOA).
  • Do not filter sterilize - the DMSO will generally kill microbes, and will dissolve standard syringe filters. We haven't had any trouble with contamination from non-filtered 5-FOA.
  • Solution can be stored at -20C, but it's generally worth just mixing it up as needed.