At present, various bioinformatic tools have allowed 25 selenocysteine-containing mammalian proteins, i.e., proteins that contain the amino acid selenocysteine (Sec), to be identified. In terms of their functions, these selenocysteine-containing proteins are oxidoreductases of different families, including glutathione peroxidases, thioredoxin reductases, deiodinases, etc. However, the functions of more than half of the identified proteins are still unclear, including the functions of the mammalian selenoprotein V (SelV). We studied the changes in the expression rate of selV gene in mice testes during postnatal development. We have observed the presence of selV mRNA at all stages of postnatal development with the maximum level of mRNA expression during puberty, while in adult mice (8-18 months), we observed a gradual decrease in expression rate. In order to get closer to the functional role of SelV, we tried to determine the substrate specificity and enzymatic activity of this protein. It was found that SelV has both glutathione peroxidase and thioredoxin reductase activity.