Melatonin, NAS, 6-OHM, AFMK, and 5-MT treated keratinocytes decrease CPD formation after UVB exposure. HEKn keratinocytes were treated with melatonin or its derivative for 24 h before UVB exposure. Cells were exposed to UVB intensities of 25 or 50 mJ/cm2 and immediately treated again with melatonin or its derivatives for 3 h. Cells were further fixed and stained with anti-CPD antibody (green) and propidium iodine (PI) (red) (A). Stained cells were imaged with a fluorescence microscope, fluorescence intensity was analysed using ImageJ software, and data are analysed using Graph Pad Prizm after 25 mJ/cm2 (B) and 50 mJ/cm2 (C). Data are presented as percentile of control and were analyzed using t-test, * p<0.05, ** p<0.01, *** p<0.001