 Mammalian GPx1, GPx2, GPx3, and GPx4 (this protein) have been shown to be selenium-containing enzymes, whereas GPx6 is a selenoprotein in humans with cysteine-containing homologues in rodents.In selenoproteins, the 21st amino acid selenocysteine is inserted in the nascent polypeptide chain during the process of translational recoding of the UGA stop codon.GPx4 shares the amino acid motif of selenocysteine, glutamine, and tryptophane (catalytic triad) with other glutathione peroxidases.