Selection Marker thymidine kinase

Targeting vector A vector containing a positive selection marker (e.g., neomycin-resistance gene), negative selection marker (thymidine kinase gene) and a multiple cloning site. 

The establishment by Littlefield (1964) of a selective system, analogous to that used in microbial genetics, for the detection of rare hybrids between cell lines carrying different drug-resistance markers, correlated with deficiencies in the enzymatic pathways required for phosphorylation of preformed thymidine and hypoxanthine. Cell lines characterized by deficiency of thymidine kinase or ino-sinic acid pyrophosphorylase are therefore unable to survive in the presence of aminopterin (an inhibitor of the de novo synthesis of purines and pyrimidines) even if the appropriate bases are supplied in the medium, while hybrid cells, restored to the full complement of enzymes, grow normaDy. 

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