Thymine, also known as 5-methyluracil, is a pyrimidine nucleobase. As the name implies, thymine may be derived by methylation of uracil at the 5th carbon. Thymine is found in the nucleic acid DNA. 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.
Thymine combined with deoxyribose creates the nucleoside deoxythymidine, which is synonymous with the term thymidine. Thymidine can be phosphorylated with one, two or three phosphoric acid groups, creating respectively TMP, TDP or TTP (thymidine mono- di- or triphosphate).
One of the common mutations of DNA involves two adjacent thymines or cytosine, which in presence of ultraviolet light may form pyrimidine dimers, causing "kinks" in the DNA molecule that inhibit normal function.