Three-Family SU(5) Grand Unification in String Theory
  We present two 3-family SU(5) grand unified models in the heterotic string theory. One model has 3 chiral families and 9 pairs of $5+{\overline 5}$ Higgs fields, and an asymptotically-free SU(2) X SU(2) hidden sector, where the two SU(2)s have different matter contents. The other model has 6 left-handed and 3 right-handed 10s, 12 left-handed and 9 right-handed ${\overline 5}$s, and an asymptotically-free SU(3) hidden sector. At the string scale, the gauge couplings $g^2$ of the hidden sector are three times as big as that of SU(5). In addition, both models have an anomalous U(1). 
Zurab Kakushadze and S.-H. Henry Tye