Comparison of Phenotypic Expression with Genotypic Transformation by Using Cloned, Selectable Markers

The frequency of phenotypic expression of the herpes simplex virus type 1 tk and Escherichia coli gpt genes was compared with the frequency of genotypic transformation after calcium phosphate-mediated DNA transfection of a number of tk- and hprt- cell lines. In three of the five lines tested, the frequency of phenotypic expression was at most 10-fold higher than that of genotypic transformation as indicated by frequency of HAT resistance. The remaining two lines showed phenotypic responses which were 50- to 100-fold greater than the genotypic responses. The data indicate that the efficiency of DNA-mediated transformation with some cell lines can be limited by events after the uptake and expression of transfected DNA.