Thymine- 13 C 5 , 15 N 2 (5-Methyluacil- 13 C 5 , 15 N 2 ) is 13 C and 15 N labeled Thymine. Thymine is one of the four nucleobases in the nucleic acid of DNA and can be a target for actions of 5-fluorouracil (5-FU) in cancer treatment, with a K m of 2.3 μM.