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Output tracking control of non-minimumphase or critically minimumphase systems is a highly challenging problem encountered in the control of flexible manipulators, space structures and elsewhere. In this paper, a class of nonlinear multi input-multi output non-minimumphase or critically minimumphase cascade systems have been considered. By defining the tracking error, the tracking problem is reduced to a regulation problem and a time-varying system is obtained. It is shown that this time-varying system can be divided into two parts, a time-invariant part and a time-varying part. With the smooth stabilizability assumption of the time-invariant part, the discontinuous control law, u1, is obtained via the backstepping procedure. It is shown that in the discontinuous points of u1, the internal dynamics are input-to-state stable, so they are bounded. Therefore, they can be ignored in the design of control law u2. Finally, it is illustrated that this discontinuous time-varying feedback law, u, includes u1 and u2 guarantees the asymptotic stability of the tracking error and the boundedness of the zero dynamics.