A total of 8 × 105 p53RMCE/+ ES cells were grown without puromycin for 12 h, electroporated with 15 μg CMV-Cre plasmid (pOG231) and 200 μg of the exchange construct, and plated in T25 flasks at 105 cells per flask. FIAU was added to the medium 3–4 days after electroporation. Individual clones, picked 10 days after electroporation, were grown in 96-well plates and expanded to generate duplicate plates for freezing and DNA analysis by PCR and Southern.