The flavour puzzle from an orbifold GUT perspective
  Neutrino masses and mixings are very different from quark masses and mixings. This puzzle is a crucial hint in the search for the mechanism which determines fermion masses in grand unified theories. We study the flavour problem in an SO(10) GUT model in six dimensions compactified on an orbifold. Three sequential families are localized at three branes where SO(10) is broken to its three GUT subgroups. Their mixing with bulk fields leads to large neutrino mixings as well as small mixings among left-handed quarks. The small hierarchy of neutrino masses is due to the mismatch between up-quark and down-quark mass hierarchies. 
Wilfried Buchmuller