Chinese hamster ovary (CHO) cells possess many advantages that make them among the most widely used and highly characterized mammalian cell lines. They are easy to culture, they grow rapidly without density dependence, and they do not have stringent culture medium or high serum growth requirements. CHO cell lines have a high plating efficiency, and many lines can grow either as a monolayer or in suspension. These features, and techniques for replica-plating CHO cells from culture dishes onto agar plates, have facilitated the isolation of mutant CHO strains, such as mutants that are UV-sensitive (1,2), X-ray-sensitive (1,3), DNA-repair-deficient (1), glycosylation-deficient (4,5), and have high sister chromatid exchange rates (6). The isolation of mutants has led to the identification of human genes that complement the defective cell function and have been valuable to investigations of DNA damage and repair, recombination, cellular transformation, and protein compartmentalization. Many of these studies have utilized transfection technology to introduce selectable vectors and genes into CHO cell lines.