  We discuss the low energy implications of gauge coupling unification at the
string scale, taking into account string threshold corrections in the $SU(4)
{\times} O(4)$ model. We express $sin^2\theta_W$ and $a_3$ as functions of the
calculable string threshold differences and discuss simple examples of spectra
which retain the successful predictions of the supersymmetric unification.
Using further the low energy data and reasonable values of the common gauge
coupling at the string scale, we obtain the range of the threshold corrections.
Finally, we study the top Yukawa coupling ($h_t$) evolution whose initial value
is determined in terms of the common gauge coupling at the string scale. We
find that $h_t$ reaches its (quasi) infra-red fixed point at the weak scale and
discuss the implications on the top mass.
