30 min. before exposure to UVB. At 24 h after irradiation, the cells were incubated for 1 h with 100 ll MTT solution (0.1 mg/ml). The tetrazolium salt (MTT) [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl- 2H-tetrazolium bromide] is reduced to a colored compound (formazan) only by living cells, due to the mitochondrial hydrogenase activity. Absorbance was read with a Tecan Sunrise microplate reader at 492 nm. Survival curves were plotted and IC50 (inhibitory
concentration 50) for BM and UVB respectively were calculated.