Uracil is one of the four RNA bases, replacing thymine as found in DNA. Just like thymine, uracil can form a base pair with adenine via two hydrogen bonds, but it lacks the methyl group present in thymine. Uracil, in comparison to thymine, will more readily degenerate into cytosine.

Uracil is also known as 2-oxy-4-oxy pyrimidine.