A D-brane alternative to unification
  We propose a minimal embedding of the Standard Model spectrum in a D-brane configuration of type I string theory. The SU(3) color and SU(2) weak interactions arise from two different collections of branes. The model is neither grand unified nor supersymmetric but it naturally leads to the right prediction of the weak angle for a string scale of the order of a few TeV. It requires two Higgs doublets and guarantees proton stability. 
I. Antoniadis, E. Kiritsis, T. Tomaras