  From the string partition function, we discuss the mass-shell and GSO
projection conditions valid for Kaluza-Klein (KK) as well as massless states in
the heterotic string theory compactified on a nonprime orbifold. Based on the
obtained conditions we construct a 4D string standard model, which is embedded
in a 6D SUSY GUT by including KK states above the compactification scale. We
discuss the stringy threshold corrections to gauge couplings, including the
Wilson line effects.
