Asymmetric Orbifolds and Grand Unification
  We generalize the rules for the free fermionic string construction to include other asymmetric orbifolds with world-sheet bosons. We use these rules to construct various grand unified string models that involve level-3 current algebras. We present the explicit construction of three classes of 3-chiral-family grand unified models in the heterotic string theory. Each model has 5 left-handed, and two right-handed families, and an adjoint Higgs. Two of them are SO(10) and the third is E_6. With Wilson lines and/or varying moduli, we show how other 3-family grand unified models, such as SO(10) and SU(5), may be constructed from them. 
Zurab Kakushadze and S.-H. Henry Tye