  We consider the physics of an extra $U(1)$ gauge boson $Z'$, which can mix
with $Z$ through intermediate fermion loops. The loop contribution due to the
heavy top quark significantly affects the low-energy observables, and for
$m_{Z'}>m_Z$, one can always adjust the shifts in these observables to be in
the right direction suggested by experiments, when we impose the anomaly
cancellation conditions for $Z'$.
