Uracil is a common and naturally occurring pyrimidine nucleobase in which the pyrimidine ring is substituted with two oxo groups at positions 2 and 4. Found in RNA, it base pairs with adenine and replaces thymine during DNA transcription. It has a role as a prodrug, a human metabolite, a Daphnia magna metabolite, a Saccharomyces cerevisiae metabolite, an Escherichia coli metabolite, a mouse metabolite and an allergen. It is a pyrimidine nucleobase and a pyrimidone. It is a tautomer of a (4S)-4-hydroxy-3,4-dihydropyrimidin-2(1H)-one.