Apoptosis assay was performed using Annexin V/FITC Kit (BD Biosciences, Catalog no. 556547), and the fluorescence intensities of FITC-conjugated annexin-V and Propidium iodide (PI) in cells were analyzed using flow cytometry. HeLa and H357 cells (1×106 cells/well) were seeded in a 6-well plate. The cells were allowed to adhere for 12 hrs, cultured in medium containing different concentrations of mangosteen extract for 48hr. The cells were then collected and washed twice with Phosphate buffer Saline, gently resuspended in 100μL annexinV-FITC binding buffer (1x) and incubated with 5μL annexinV-FITC in the dark for 10 min at 25°C. This was followed by centrifugation of cells at 2000 rpm for 5 min, and gently resuspended in 500μL annexinV-FITC binding buffer (1x) and 5μL PI was added in an ice bath, followed by immediate analysis by flow cytometry Cell Quest software (BD Biosciences).