MSRB1 (methionine sulfoxide reductase B1) is a selenoprotein that catalyzes the stereospecific reduction of methionine (R)-sulfoxide back to methionine 1. As a member of the selenoprotein family containing a catalytic selenocysteine residue 2, MSRB1 functions in two primary contexts: oxidative stress protection and immune regulation. Mechanistically, MSRB1 regulates actin dynamics by reducing methionine (R)-sulfoxide modifications catalyzed by MICAL enzymes on actin, thereby promoting filament repolymerization 3. In immune cells, lipopolysaccharide induces MSRB1 expression in macrophages, where it promotes anti-inflammatory cytokine production (IL-10 and IL-1 receptor antagonist) while suppressing excessive pro-inflammatory responses 4. MSRB1 expression is upregulated by zinc, which protects lens epithelial cells against oxidative stress 5. Disease relevance extends to colorectal cancer, where MSRB1 acts as an oncogene 2. The transcription factor KLF5 activates the MSRB1 promoter, and MSRB1 cooperates with β-catenin to activate GPX4, inhibiting ferroptosis and promoting cancer progression 2. MSRB1 involvement in cognition, neuronal maintenance, and cancer proliferation has been demonstrated in transgenic models 1. These findings establish MSRB1 as both a protective selenoprotein against oxidative damage and a potential oncogenic target in colorectal cancer.