Use of fluorescence-activated cell sorter to isolate mutant mammalian cells deficient in an internal protein, dihydrofolate reductase.

Dihydrofolate reductase-deficient mutants of Chinese hamster ovary cells have been selected based on their inability to bind a fluorescent derivative of methotrexate, a substrate analog. Nonfluorescent mutant cells were isolated from mutagenized populations using a fluorescence-activated cell sorter. After multiple rounds of sorting plus regrowth, the mutant cell frequency was increased from an initial 10(-5) to greater than 0.9. This use of the cell sorter to isolate mutants deficient in an internal protein should be applicable to any gene product that is able to bind a fluorescent ligand tightly. The method has the advantage of allowing the screening of large numbers of cells and of selecting for partially expressed phenotypes.