Thymidine kinase is an alias for the gene, Thymidine kinase 1, soluble, in humans. Other names include: Thymidine kinase, cytosolic ; TK2. The encoded protein is an enzyme with an amino acid length of 234 and a mass of 25.5 kDa. Other isoforms may exist. Thymidine kinase is a member of the Thymidine kinase family. This gene shares homology with other species, including: Mouse, Rat.