Deconstruction, G_2 Holonomy, and Doublet-Triplet Splitting
  We describe a mechanism for using discrete symmetries to solve the doublet-triplet splitting problem of four-dimensional supersymmetric GUT's. We present two versions of the mechanism, one via ``deconstruction,'' and one in terms of M-theory compactification to four dimensions on a manifold of G_2 holonomy. 
Edward Witten