A hadron with half-integral spin. Nucleons comprise a subclass of baryons. According to currently accepted theory, baryons are made up of three quarks (antibaryons are made up of three antiquarks) held together by gluons (see quantum chromodynamics). Baryons possess a quantum number, called the baryon number, which is +1 for baryons, −1 for antibaryons, 1/3 for quarks, −1/3 for antiquarks, and 0 for all other particles such as electrons, neutrinos, and photons. Baryon number has always appeared to have been conserved experimentally, but grand unified theories postulate interactions at very high energies that allow it not to be conserved. It is thought that nonconservation of baryon number at the high energies characteristic of the early universe is necessary for an explanation of the asymmetry between matter and antimatter in the universe. See proton decay.