2. Add thymidine to a final concentration of 2 mM for 16-24 hours (one cell cycle).

3. Wash cells with PBS three times and re-feed with fresh grow medium.

4. Incubate for 10 hours before adding thymidine to a final concentration of 2 mM.

5. Incubate for 14-22 hours.

6. Repeat step 3. This will be time 0.