We consider the possibility of breaking a rank-six superstring model to the rank-four standard model. In particular, we point out the difficulties in generating two vacuum expectation values for the two standard model singlets contained in the 27 of E6. Although one expectation value is compatible with low energy phenomenology, a vev for vc is problematic because of the absence of large neutrino masses and / or flavor changing neutral currents. We show that even simple models containing extra fields from incomplete multiplets or E6 singlets do not resolve these problems.
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We would like to thank John Ellis for a criticalr eadingo f this manuscriptO. ne of us (B.A.C.) would like to thank the Universityo f Minnesotaa nd the CERN Theory Division for their kind hospitalityd uringthe completiono f this work. His research was supportedb y NSERC Canada under grant A8863. The work of K.A.O. and D.B.R. was supportedin part by DOE grant DE-AC02-83ER-40105.