Selenium protects normal cells from DNA damage while enhancing DNA damage in malignant cells. (a) Representative image of comet assay (400× magnification) in PBMCs and THP1 cells treated with MSA alone or MSA in combination with radiation, AraC or Dox; (b) Quantification of DNA damage in PBMCs and THP1 cells: treatment with MSA 15 µM, but not lower concentrations, slightly increased DNA damage levels in normal but not in malignant cells; (c) DNA damage is increased in both PBMCs and THP1 cells exposed to 2 Gy radiation; (d,e) MSA is significantly protective against DNA damage in PBMCs while significantly increasing DNA damage in THP1 cells treated with AraC or Dox. n = 3, ± SEM, ** p < 0.01, *** p < 0.001, ns, not significant.