Assessment of CLS using PI was performed as previously described (Choi et al., 2013 (link); 2015 (link); Kwon et al., 2015 (link)). Cells were harvested by centrifugation, resuspended with 1 ml phosphate-buffered saline (PBS) for washing, and incubated for 20 min at 30°C after adding 5 μl of PI solution (1 mg/ml, Sigma Aldrich, USA). Stained cells were analyzed with flow cytometry (FACS Verse; Becton Dickinson, USA). Excitation was performed using a blue laser at 488 nm and emission was detected at 585 nm in the FL2 channel. A total of 20,000 cells were analyzed for each sample, and data was analyzed using Cell Quest software (Becton Dickinson).