An enzyme which plays an important role in the making of ATP and GTP from guanine -- the only way guanine, adenine, or other purine 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 thymidine kinase is used for the same purpose by biologists because it fills the same important and unique role for the pyrimidines (thymine, cytosine, uracil, etc.).