In supersymmetry, it is proposed that every fermion should have a "partner" boson, known as its Superpartner. No superpartners have yet been found, but it is hypothesized that they are only available at extremely high energy level. Discovery of such a particle is considered essential to proving Superstring theories or M-Theory. The addition of Supersymmetry to String Theory allowed it to describe fermions in addition to bosons, since the bosons described in string theory would necessarily have fermion superpartners.