RO3306 as a potent CDK1 inhibitor inhibited the cell growth and colony formation of HEC-1-B cells in vitro. (A, B) HEC-1-B cells were treated with RO3306 at different concentrations of 0, 2.5, 5, 10, 15 and 20 µM. Cell viability was determined at 24, 48, and 72 hours by a CCK-8 assay. Cell viability values are expressed relative to those for cells without drug exposure. Data were presented as means ± SD (n = 3). (A) Time-response effect of RO3306 on HEC-1-B cells. (B) Dose-response effect of RO3306 on HEC-1-B cells. The IC50 value for RO3306 at 72 h were obtained by GraphPad Prism version 7.0 software. (C) Colony formation assay. HEC-1-B cells were treated with RO3306 at different concentrations of 0, 2.5 and 5 µM for 48 h. Cells were fixed and stained at the end of the study.