1‐Cys peroxiredoxin (1‐cysPrx), a member of the peroxiredoxin family with a single conserved cysteine, is a unique antioxidant enzyme. We have generated mice in which the 1‐cysPrx gene has been inactivated; they are viable and fertile. Mice have a highly related intronless gene (1‐cysPrx‐P1, GenBank accession number AF085220) with the same length of open reading frame (224 aa) as 1‐cysPrx but located on a different chromosome. Since the product of this gene possibly could mimic 1‐cysPrx function, we compared expression of 1‐cysPrx and 1‐cysPrx‐P1 in mouse tissues by real‐time polymerase chain reaction and Western blot. 1‐cysPrx mRNA and protein were expressed in all mouse tissues that were examined with the highest expression level in lung. 1‐cysPrx‐P1 mRNA was expressed only in testis. In the 1‐cysPrx ‘knock‐out’ mouse, 1‐cysPrx‐P1 mRNA expression level was similar to the wild type but protein expression was not detected. Thus, mouse 1‐cysPrx‐P1 is an mRNA‐expressed pseudogene that does not result in detectable protein in vivo.