 While mammals have only one copy of the GPX4 gene, fish have two copies, GPX4a and GPX4b.The GPX4's appear to play a greater role in the fish GPX system than in mammals.For example, in fish GPX4 activity contributes to a greater extent to total GPX activity, GPX4a is the most highly expressed selenoprotein mRNA (in contrast to mammals where it is GPX1 mRNA) and GPX4a appears to be highly inducible to changes within the cellular environment, such as changes in methylmercury and selenium status.