Cys deficiency in vitro results in substantial oxidation of GSH/GSSG redox. In a study of Cys deficiency and readdition of Cys in colon carcinoma HT29 cell line, we found that culture in medium with low Cys and CySS resulted in decreased GSH and GSSG with an associated 80 mV oxidation of the GSH/GSSG redox state (29). Upon addition of either Cys or CySS, redox of GSH/GSSG recovered in 1 h while GSH concentration continued to increase over 8 h (33). These results have now been confirmed in Hela cells and normal human retinal pigment epithelial cells (not shown) indicating that variation in Cys availability to cells has a general effect on GSH/GSSG redox state.