SEPX1, also known as Methionine sulfoxide reductase B1 (MSRB1), is a selenoprotein. Methionine sulfoxide reductases(MSRs) catalyze reduction of free and protein-bound methionine sulfoxides to corresponding methionines. The oxidation of methionine by ROS generates a diastereomeric mixture of methionine-S-sulfoxide (Met-S-SO) and methionine-R-sulfoxide (Met-R-SO). Two distinct enzyme families evolved for reduction of these sulfoxides, with methionine-S-sulfoxide reductase (MsrA) being stereospecific for Met-S-SO and methionine-Rsulfoxide reductase (MsrB) for Met-R-SO. In bacteria, the selenocystein (Sec/U) element is located immediately following the UGA codon within the reading frame for the selenoprotein so we mutated Sec-95 to Cys. Recombinant human SEPX1 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.