Abstract General conditions for the suppression of proton decay mediated by gauge boson exchange are considered. One appealing way to satisfy these conditions is to delay the unification of left-handed with right-handed quarks. The simplest grand unified theories which have a two-stage breaking of the gauge group to SU(3) × SU(2) × U(1) are discussed. A general analysis of gauge mediated ΔB ≠ 0 interactions is given for the various ways of embedding SU(6) × SU(2) × U(1) in the maximal subgroup of E 6. A model based on the SU(6) × SU(2) R subgroup of E 6 is considered in detail, and is shown to give a proton lifetime up to a few thousand times longer than in the minimal SU(5) theory.