Sigma 3 is the major outer capsid protein of reovirus. Sigma 3 is encoded by genome segment 4. Sigma 3 binds to double stranded RNA and associates with polypeptide u1 and its cleavage product u1C to form the outer shell of the virion. The Sigma 3 protein possesses a zinc-finger motif and an RNA-binding domain in the N and C termini respectively. This protein is also thought to play a role in pathogenesis.