Inhibition of thymidine phosphorylase in vivo provides a rapid method for switching DNA labeling.

Uridine blocks the in vivo conversion of thymine to thymidine in Escherichia coli, thus, one can change DNA labels by labelling first with a thymine label (e.g. 14C) and then, at the time of the change, adding 50 microgram uridine per ml and thymidine (e.g. 3H). The cells immediately start using the thymidine, ignore the thymine for several generations, and are not affected by the uridine.