The Search for a realistic flipped SU(5) string model
  We present an extensive search for a general class of flipped $SU(5)$ models built within the free fermionic formulation of the heterotic string. We describe a set of algorithms which constitute the basis for a computer program capable of generating systematically the massless spectrum and the superpotential of all possible models within the class we consider. Our search through the huge parameter space to be explored is simplified considerably by the constraint of $N=1$ spacetime supersymmetry and the need for extra $Q,\bar Q$ representations beyond the standard ones in order to possibly achieve string gauge coupling unification at scales of ${\cal O}(10^{18}\GeV)$. Our results are remarkably simple and evidence the large degree of redundancy in this kind of constructions. We find one model with gauge group $SU(5)\times U(1)_\ty\times SO(10)_h\times SU(4)_h\times U(1)^5$ and fairly acceptable phenomenological properties. We study the $D$- and $F$-flatness constraints and the symmetry breaking pattern in this model and conclude that string gauge coupling unification is quite possible. 
J. Lopez, D. Nanopoulos, and K. Yuan