  We investigate supersymmetric $SU(4)_c\times SU(4)_{L+R}$ theory in 5
dimensions whose compactification on a $S^{(1)}/Z_2$ orbifold yields N=1
supersymmetric $SU(4)_c\times SU(2)_L\times SU(2)_R$ supplemented by a
$\tl{U}(1)$ gauge symmetry. We discuss how the $\mu $ problem is resolved, a
realistic Yukawa sector achieved, and a stable proton realized. Neutrino masses
and oscillations are also briefly discussed.
