Methionine Sulfoxide Reductase B (MSRBs) are able to reduce methyl-(R)-S-oxide to methionine (Grimaud et al. 2001). They are specific for the reduction of protein-based methyl-(R)-S-oxide, reducing free methyl-(R)-S-oxide with very low efficiency (Lee et al. 2009). Mammals have at least 3 MSRB genes (Kryukov et al. 1999, Huang et al. 1999, Jung et al. 2002, Kim & Gladyshev 2004). They are ubiquitously expressed, no clear substrate specificities are known, all three contain a zinc atom and can use thioredoxin as an in vivo reducing agent (Kim & Gladyshev 2007).