SU(5) is the only choice for a unifying algebra that makes use only of the matter particles that we actually see. However, it is not obvious that this is a necessary property. We know that the unifying symmetry, whatever it is (and assuming that it exists at all!) must be spontaneously broken to account for the differences in the interactions that we see. It could be that the process of spontaneous breaking gives a large mass to some of the matter particles of a representation, thus effectively removing them. The next simplest unification, based on the algebra SO(10), adds just one such matter particle - a right-handed neutrino.