Levels of CDKs and CDK inhibitors are not affected under conditions of thiol deprivation. Western blot analyses of (A) p27^Kip1 and (B) CDK6/cyclin D3 and CDK2 and cyclin E in NK3.3 cells revealed little dependency on medium thiols. NK3.3 cells were depleted of residual thiols and then stimulated with IL-2 (200 U/mL) in CYS(+) or CYS(−), and lysed at the indicated time points. Each figure is representative of at least two independent experiments. Experiments performed with different quantities of cell extracts yielded consistent results.