Selenoproteins contain the rare amino acid selenocysteine (Sec). There are 25 genes encoding selenoproteins in humans and 24 in mice (Kryukov et al. 2003). Despite the small number of selenoproteins, they are important for many biological processes, and some selenoproteins are essential for life. Accordingly, mutations in selenoproteins, or in any of the genes required for selenoprotein biosynthesis, lead to pathology in humans carrying such mutations, as well as in mouse models designed to be deficient for these genes (Schweizer & Fradejas-Villar 2016).