Selenoprotein X1 (SEPX1)

Methionine-R-sulfoxide reductase B1 is a selenoprotein, which contains a selenocysteine (Sec) residue at its active site. The selenocysteine is encoded by the UGA codon that normally signals translation termination. The 3' UTR of selenoprotein genes have a common stem-loop structure, the sec insertion sequence (SECIS), that is necessary for the recognition of UGA as a Sec codon rather than as a stop signal.

This protein belongs to the methionine sulfoxide reductase B (MsrB) family, and it is expressed in a variety of adult and fetal tissues.The deduced 116-amino acid protein has a calculated molecular mass of 12.6 kD. The SECIS element is located in the 3-prime untranslated region, 554 nucleotides downstream of the sec codon.