Methionine sulfoxide reductase B1 （SEPX1 or MSRB1）， is a selenoprotein that contains a selenocysteine （Sec） residue at its active site. The selenocysteine is encoded by the UGA codon that usually signals translation termination. SEPX1 is a member of the methionine sulfoxide reductase B （MsrB） family， and is expressed in an assortment of adult and fetal tissues. MSRs （Methionine sulfoxide reductases） catalyze the reduction of free and protein-bound methionine sulfoxides to corresponding methionines. The oxidation of methionine by ROS creates a diastereomeric mixture of methionine-S-sulfoxide （Met-S-SO） and methionine-R-sulfoxide （Met-R-SO）. Two separate enzyme families evolved for reduction of these sulfoxides， with methionine-S-sulfoxide reductase （MsrA） being stereospecific for Met-S-SO and methionine-R-sulfoxide reductase （MsrB） for Met-R-SO.
