Peroxiredoxins catalyze reduction of hydrogen peroxide or alkyl peroxide, to water or the corresponding alcohol. Detailed analysis of their sequences indicates that these enzymes possess a thioredoxin (Trx)-like fold and consequently are homologues of both thioredoxin and glutathione peroxidase (GPx). Sequence- and structure-based multiple sequence alignments indicate that the peroxiredoxin active site cysteine and GPx active site selenocysteine are structurally equivalent. Homologous peroxiredoxin and GPx enzymes are predicted to catalyze equivalent reactions via similar reaction intermediates.