1.   Culver said the idea was to use thymidine kinase as a way to destroy the genetically engineered cells if the ADA experiment went awry.

2.   In modifying vaccinia for use against rabies, French and American scientists snipped a gene called thymidine kinase from the genetic code of one of the vaccinia virus strains.

3.   One of the viral genes instructs the cells to make an enzyme, thymidine kinase, that normally would be completely innocuous.

4.   The herpes gene produces its own enzyme, known as thymidine kinase.

5.   The gene codes for the enzyme thymidine kinase, and this enzyme, in turn, enables the virus to replicate its DNA.