Baryons are hadrons containing an odd number of valence quarks (at least 3). Most well known baryons such as the proton and neutron have three valence quarks, but pentaquarks with five quarks – three quarks of different colors, and also one extra quark-antiquark pair – have also been proven to exist. Because baryons have an odd number of quarks, they are also all fermions, i.e., they have half-integer spin. As quarks possess baryon number B = 1 ⁄ 3, baryons have baryon number B = 1. Pentaquarks also have B = 1, since the extra quark's and antiquark's baryon numbers cancel.