Strings versus supersymmetric GUTs: can they be reconciled?
  We describe a class of supersymmetric unified models with the following properties:   i) the full breaking of the gauge group is achieved by Higgs fields in the fundamental representation;   ii) the correct unification of the strong and electroweak coupling constants is obtained without the need of any intermediate scale;   iii) the problems of the doublet-triplet splitting and of the proton decay at dimension-5 level may receive a natural solution.   The models, other than being interesting unified field theories per se, may constitute examples of string-derivable GUTs. 
R. Barbieri, G. Dvali and A. Strumia