The MDA-MB-231, SUM149 and SUM159 cell lines (5x104 cells/well) were seeded in 6-well plates and treated with GO (IC50: 3.78, 1.85 and 1.60 mmol/l, respectively) for 24 h. Next, the cells were collected and stored in pre-cooled alcohol overnight at 4˚C, then stained with PI (Shanghai Yeasen Biotechnology, Co., Ltd.) for 15 min in the dark at 4˚C. The samples were tested using a Guava EasyCyte Plus flow cytometer (Merck KGaA) and FlowJo VX (Becton-Dickinson and Company) was used to analyze the results. The test was repeated three times.