inhibit copper-mediated DNA damage (see supplementary material for gel images and inhibition data). The amino acids methionine and methyl-cysteine have IC50 values higher than the disulfides cystine or oxidized glutathione (11.8 ± 1.3 and 9.6 ± 1.0 lM, respectively, Table 1), and inhibit > 95% of DNA damage at 100 lM. 2-Aminophenyl disulfide, cystamine, 3-carboxypropyl disulfide, and 2,20-dithiosalicylic acid do not inhibit copper-mediated DNA damage.