PERIOD2 is a key substrate of CK1ε that mediates IC261-induced growth inhibition. (a) IC261, a kinase inhibitor of CK1ε, induces growth inhibition in HT1080 cells. (b) IC261 treatment in BJ-derived cell lines showed a similar degree of cancer cell selective growth inhibition as shCSNK1E treatment. (c) Knocking down PER2 in HT1080 cells rescues growth inhibition induced by IC261. HT1080 cells were infected with indicated lentiviruses containing different shRNA clones targeting PER2 (per2_538, per2_539, per2_541, or per2_542). After two days of infection, cells were treated with the indicated concentration of IC261 and percent growth inhibition was determined using alamar blue. Values in (a-c) represent the mean ± standard deviation of triplicate data. (d) Cellular RNAs were prepared from the same set of virus infected cells in (c), and real-time PCR was performed with a PER2-specific primer set to monitor the efficiency of knock down by shRNA clones. (e) Proliferation rate of HT1080 cells infected with the same set of viruses as in (c) was determined using alamar blue assay. Error bars in (d,e) indicate one standard deviation of triplicate data. N.T., non-targeting shRNA clone.