A well known problem in supersymmetric models is the presence of, lepton and baryon number violating, dimension four operators. The traditional R parity solution may not be suitable if one tries to incorporate supersymmetry into a Planck scale theory. I propose a different solution in the context of realistic string models. I show that realistic string models can give rise to custodial nonabelian gauge symmetries under which only the leptons or quarks transform. I explain how such symmetries

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... se in a class of free fermionic models that are based on Z_2× Z_2 orbifold with standard embedding. The custodial symmetries forbid proton decay from dimension four operators while allowing R parity violation.