  Assuming that the Standard Model of particle physics arises from the
$E_8\times E_8$ Heterotic String Theory, we try to solve the discrepancy
between the unification scale predicted by this theory ($\approx g_{GUT}\times
5.27\cdot 10^{17}$ GeV) and the value deduced from LEP experiments ($\approx
2\cdot 10^{16}$ GeV). A crucial ingredient in our solution is the presence at
low energies of three generations of supersymmetric Higgses and vector-like
colour triplets. As a by-product our analysis gives rise to a strategy which
might be useful when constructing realistic models.
