A polyribosome (or polysome) is a complex of a mRNA molecule and two or more ribosomes that is formed during active translation. Originally coined "ergosomes" in 1963 they were further characterized by Jonathan Warner, Paul Knopf, and Alex Rich. Polysomes can be directly visualized by electron microsopy because they form very high molecular weight particles. Many ribosomes simultaneously read one mRNA progressing along the mRNA to synthesize the same protein.