Abstract
Cell lines that have high levels of resistance to .beta.-aspartylhydroxamate and elevated levels of asparagine synthetase activity were selected in 2 steps from Chinese hamster ovary cells. Resistance to .beta.-aspartylhydroxamate was transferred into sensitive cells by using total genomic DNA derived from the dominant 2-step mutants. The surviving colonies were characterized as transferants on the basis of transfer frequency, degree of resistance to .beta.-aspartylhydroxamate, increased level of asparagine synthetase activity, expression of the donor form of asparagine synthetase, codominance in hybrids and instability of the phenotype in the absence of selection.