Ad.CD virus expresses functional cytosine deaminase. HeLa cells were transduced with Ad.CD at an MOI of 50 and incubated for 12 h. These cells (HeLa/CD) were then mixed with naïve HeLa cells at the indicated ratios. The HeLa/CD, HeLa, or mixed cells were incubated on 24-well dishes for 24 h, then treated with 5-FC at the indicated concentrations for 4 days. Cell viability was assessed by the trypan-blue exclusion assay. Data are means ± SD of two independent experiments, each performed in duplicate.