While the majority of SelP is expressed in the liver of adult mammals, SEPP1 orthologs in fish and mammals demonstrate broad tissue expression. Zebrafish, who have an extensive selenoproteome, includes two SEPP1 isoforms encoded by distinct genes; one (sepp1a) with a selenocysteine-rich C-terminus containing 16 selenocysteine residues, and a second isoform (sepp1b) that lacks the selenocysteine-rich C-terminus (Kryukov & Gladyshev 2000). These genes demonstrate distinct spatiotemporal expression patterns throughout the development of the zebrafish with sepp1a displaying expression in multiple organs including the heart, brain and kidney, but only limited hepatic expression, while sepp1b demonstrates strong hepatic expression (Thisse et al. 2003). In addition, a recent study of the expression of the murine ortholog of SEPP1 in mouse embryos also highlights a potential role of SelP in growth and developmental processes. Spatiotemporal expression of Sepp was observed in the central nervous system, limb buds, blood cells, lung, liver, intestine, testis, and developing epithelia, as well as in extraembryonic tissues, during organogenesis. The authors suggest that this increase in Sepp may provide antioxidant protection against the reactive oxygen species formed during
