To assess the effects of cisplatinum and light on cell viability, an XTT assay was performed, following the manufacturer's instructions (Cell Proliferation Kit II XTT; Merck KGaA). Briefly, the A431 and HaCaT cells were seeded in 96-well plates (Corning Inc.) at a final concentration of 1.6×104 cells/well and exposed to a cycle of 12L:12D (blue or red light) for 3 days; constant darkness was used as control. Subsequently, the appropriate concentration of cisplatinum for each cell line was added. At 24 h, 50 μl XTT (2,3-bis-(2-met hoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) solution was added to each well, followed by incubation for 2 h at 37°C and 5% CO2. Finally, the absorbance at 450 nm with 650 nm as the reference wavelength was measured using an absorbance microplate ELISA plate reader (Sunrise™ Absorbance Reader; Tecan Group Ltd.).