This study demonstrates the feasibility of transforming mouse cells deficient in adenine phosphoribosyltransferase (aprt) to the aprt+ phenotype using DNA-mediated gene transfer. The transformation was achieved using unfractionated high molecular weight genomic DNA from various species, such as Chinese hamster, hamster, and mouse cells, along with restriction enzymes like HincII and HpaII. The transformation frequency was between 1 and 10 colonies per 10^6 cells per 20 μg of donor DNA. Transformants displayed enzymatic activity derived from the donor, as confirmed by isoelectric focusing of cytoplasmic extracts. These transformants were categorized into two groups: those that remained phenotypically stable under selective pressure and those that were phenotypically unstable. The study also explored the use of restriction endonuclease-cleaved DNA for transformation, finding that cleavage with Kpn I or HindIII did not reduce transformation efficiency compared to uncut DNA. The results indicate that DNA-mediated gene transfer can be a powerful tool for studying gene function and facilitating the isolation of genes from animal cells.This study demonstrates the feasibility of transforming mouse cells deficient in adenine phosphoribosyltransferase (aprt) to the aprt+ phenotype using DNA-mediated gene transfer. The transformation was achieved using unfractionated high molecular weight genomic DNA from various species, such as Chinese hamster, hamster, and mouse cells, along with restriction enzymes like HincII and HpaII. The transformation frequency was between 1 and 10 colonies per 10^6 cells per 20 μg of donor DNA. Transformants displayed enzymatic activity derived from the donor, as confirmed by isoelectric focusing of cytoplasmic extracts. These transformants were categorized into two groups: those that remained phenotypically stable under selective pressure and those that were phenotypically unstable. The study also explored the use of restriction endonuclease-cleaved DNA for transformation, finding that cleavage with Kpn I or HindIII did not reduce transformation efficiency compared to uncut DNA. The results indicate that DNA-mediated gene transfer can be a powerful tool for studying gene function and facilitating the isolation of genes from animal cells.