The cells were collected after being cultured with neomycin (0.008 g/L) for 6 h. Cell cycle analysis was performed by fixing cells with 75% alcohol in PBS. The cells were collected by centrifugation and resuspended in PBS, then incubated in propidium iodide solution (containing PI 20 mg/L) in the darkness for 15 min. Cells were centrifuged again and the pellets were resuspended in PBS. DNA content was determined by flow cytometry.