Cytosine deaminase (EC 3.5.4.1), a non-mammalian enzyme, catalyzes the
deamination of cytosine and 5-fluorocytosine to form uracil and 5-flu
orouracil, respectively. Eukaryotic cells have been genetically modifi
ed with a bacterial cytosine deaminase gene to express a functional en
zyme, When the genetically modified cells are combined with 5-fluorocy
tosine, it creates a potent negative selection system, which may have
important applications in cancer gene therapy, In this paper, we intro
duce a novel positive selection method based upon the expression of th
e cytosine deaminase gene, This method utilizes inhibitors in the pyri
midine de novo synthesis pathway to create a condition in which cells
are dependent on the conversion of pyrimidine supplements to uracil by
cytosine deaminase, Thus, only cells expressing the cytosine deaminas
e gene can be rescued in a positive selection medium.