Mammals have two isoenzymes, that are chemically very different, TK1 and TK2. The former was first found in fetal tissue, the second was found to be more abundant in adult tissue, and initially they were termed fetal and adult thymidine kinase. Soon it was shown that TK1 is present in the cytoplasm only in anticipation of cell division (cell cycle-dependent), whereas TK2 is located in mitochondria and is cell cycle-independent. The two isoenzymes have different reaction kinetics and are inhibited by different inhibitors.