DNA-mediated gene transfer is more efficient during S-phase of the cell cycle

We transformed synchronized mouse L-thymidine kinase deficient cells with a cloned thymidine kinase gene and carrier DNA at different times in the cell cycle. The frequency of resistant colonies was determined after growth in hypoxanthine-aminopterin-thymidine medium for two weeks (late expression) and the frequency of thymidine kinase positive cells was determined 72 hours after transformation (early expression) by incorporation of 3H-thymidine and autoradiography. The frequency of late expression was several-fold higher when cells were exposed to DNA during S-phase, whereas the frequency of early expression did not depend on the phase of the cell cycle.