Figure 9. PER1 expression does not sensitize human cancer cells to ionizing radiation in a colony formation assay. Cells were transfected with indicated siRNAs or expression vectors and exposed to IR, and IR survival was measured by colony formation assay. (A) Colony formation assay results for NCI-H460 human lung cancer cells. After 48 h of siRNA treatment, cells were trypsinized and plated at low density (500 cells per dish) in 6-well culture dish plates. After allowing cells to attach for 8 h, they were irradiated with the indicated fluence of IR and allowed to grow in fresh growth media for 7–10 d. Colonies (> 50 cells) were counted using a digital counter after staining with Giemsa and the surviving cell fraction was calculated. (B) Colony-formation assay results for HCT-116 human colon cancer cells. After 48 h of siRNA treatment, cells were trypsinized and plated at low density in 6-well plates. The results are the average of three biological experiments and error bars represent the standard deviation of the mean.