Flat Directions in Flipped SU(5) I: All-Order Analysis
  We present a systematic classification of field directions for the string-derived flipped SU(5) model that are D- and F-flat to all orders. Properties of the flipped SU(5) model with field values in these directions are compared to those associated with other flat directions that have been shown to be F-flat to specific finite orders in the superpotential. We discuss the phenomenological Higgs spectrum, and quark and charged-lepton mass textures. 
G.B. Cleaver, J. Ellis, and D.V. Nanopoulos