DNA-mediated gene transfer in Chinese hamster ovary cells: clonal variation in transfer efficiency.

Thymidine kinase-deficient Chinese hamster ovary (CHO) cells were genetically transformed with the BamHI restriction fragment encoding the thymidine kinase gene of herpes simplex virus (HSV-tk). We have observed considerable clonal variation among independent CHO sublines with respect to transformation competence for the DNA-mediated gene transfer of HSV-tk. Transformation frequencies greater than or equal to 3 X 10(-4) were observed consistently in one subline, with a transformation efficiency of approximately 1 transformant per ng viral gene. The frequency and efficiency of transformation we observed in this system are at least 10-fold greater than those previously reported for DNA-mediated transformation of CHO cells by HSV-tk. All of the CHO HSV-tk+ transformants examined were stable for the transferred genotype in the absence of selection, and all showed evidence of co-transformation by unselected plasmid pBR322 sequences.