  We construct a four-dimensional SU(5) grand unified theory in which the
proton is stable. The Standard Model leptons reside in the 5 and 10 irreps of
SU(5), whereas the quarks live in the 40 and 50 irreps. The SU(5) gauge
symmetry is broken by the vacuum expectation values of the scalar 24 and 75
irreps. All non-Standard Model fields are heavy. Stability of the proton
requires three relations between the parameters of the model to hold. However,
abandoning the requirement of absolute proton stability, the model fulfills
current experimental constraints without fine-tuning.
