Because cysteine (Cys) and cystine (Cys2) are also important intracellular sulfur antioxidants, we quantified the ability of these compounds to inhibit copper-mediated DNA damage using the same method. Cys2 inhibits 100% of copper-mediated DNA damage at 10 lM and has an IC50 value of 3.4 ± 1.1 lM, demonstrating that Cys2 is an excellent antioxidant with activity almost twice that of the major cellular disulfide, GSSG. Cysteine is also a potent antioxidant, with an IC50 value of 7.6 ± 1.2 lM (Table 1). As was observed for the thiol GSH, higher concentrations of reduced Cys are required to inhibit DNA damage compared to the disulfide Cys2.