Methionine sulfoxide reductase B (MsrB), also known as SelX, is a selenoprotein. The oxidation of methionine at the sulfur atom leads to alternative epimers: R form of Met(O) and S form of Met(O). MsrB can reduce R form of both free and protein-incorporated methionine sulfoxide to methionine. It has a crucial role in protecting cells against oxidative damages. MsrA reduces only the S epimer of Met(O), and MsrB reduces the R epimer of Met(O) in proteins. Although the catalytic mechanisms of MsrA and MsrB are similar, two Msrs have no sequence identity and no structural similarity.