Thymine is one of the four bases in the nucleic acid of DNA that make up the letters ATGC. The others are adenine, guanine, and cytosine. Thymine (T) always pairs with adenine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name implies, thymine may be derived by methylation of uracil at the 5th carbon. In RNA thymine is replaced with uracil in most cases. In DNA, thymine(T) binds to adenine (A) via two hydrogen bonds to assist in stabilizing the nucleic acid structures.