||Labelled analogue of Thymine. Thymine (T, Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G-C-A-T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidinenucleobase. In RNA, thymine is replaced by the nucleobase uracil. Thymine was first isolated (from calves' thymus glands) in 1893 by Albrecht Kossel and Albert Neumann.