Cell viability assay. The effects of the grape stem extracts on cell viability were assessed using the XTT assay kit (Roche Diagnostics) as previously described (21). Briefly, the cells were subcultured in a 96-well plate with 1x104 cells per well in DMEM. After 24 h of incubation, the cells were treated with increasing concentrations of the extracts in serum-free DMEM for 24 h. A total of 50 µl of XTT test solution, which was prepared by mixing 50 µl of XTT-labeling reagent with 1 µl of electron coupling reagent, was then added to each well. After 4 h of incubation, the absorbance was measured at 450 nm and also at 690 nm as a reference wavelength in a Bio-Tek ELx800 microplate reader (Winooski, VT, USA). DMEM serum-free medium was used as a negative control. In addition, the absorbance of each grape extract concentration alone in DMEM serum-free medium and XTT test solution was tested at 450 nm. The absorbance values shown by the grape extracts alone were subtracted from those derived from cells treated with the grape extracts. Data were calculated as a percentage of inhibition using the following formula: inhibition (%) = [(OD control - OD sample)/OD control] x100, where OD control and OD sample indicate the optical density of the negative control and the tested substances, respectively. All experiments were carried out in triplicate and at least on 2 separate occasions.