  The aim of superstring phenomenology is to develop the tools and methodology
needed to confront string theory with experimental data. The first mandatory
task is to find string solutions which reproduce the observable data. The
subsequent goal is to extract potential signatures beyond the observable data.
Recently, by studying exact flat directions of non-Abelian singlet fields, we
demonstrated the existence of free fermionic heterotic-string models in which
the $SU(3)\times SU(2)\times U(1)_Y$-charged matter spectrum, just below the
string scale, consists solely of the MSSM spectrum. In this paper we study the
possibility that the exact flat directions leave a $U(1)_{Z^\prime}$ symmetry
unbroken at the Planck scale. We demonstrate in a specific example that such
unbroken $U(1)_{Z^\prime}$ is in general expected to be not of the GUT type but
of intrinsic stringy origin. We study its phenomenological characteristics and
the consequences in the case that $U(1)_{Z^\prime}$ remains unbroken down to
low energies. We suggest that observation in forthcoming colliders of a
$Z^\prime$, with universal couplings for the two light generations but
different couplings for the heavy generation may provide evidence for the
$Z_2\times Z_2$ orbifold which underlies the free fermionic models.
