An enzyme (abbreviated tk) which plays an important role in the making of thymidine triphosphate from thymine -- the only way thymine, cytosine, and other pyrimidine molecules are able to become part of nucleic acids.
Biologists study gene action via the incorporation of modified nitrogenous bases into DNA by manipulating the metabolic role which this enzyme plays. Similarly, the enzyme hypoxanthine-guanine phosphoribosyl transferase is used for the same purpose by biologists because it fills the same important and unique role for the purines (guanine, adenine, etc.). Thymidine kinase is also used as a selectable marker to identify transfected eukaryotic cells.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. Used with permission. For further information see the BioTech homenode.