Thymidine - 13 C 10 (DThyd- 13 C 10 ; NSC 21548- 13 C 10 ) is 13 C-labeled Thymidine. Thymidine , a specific precursor of deoxyribonucleic acid, is used as a cell synchronizing agent. Thymidine is a DNA synthesis inhibitor that can arrest cell at G1/S boundary, prior to DNA replication.