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A robust control is designed for a class of uncertain systems, the proposed control is distinct and novel in that it does not require any information of a bounding function on nonlinear uncertainties in the system. Instead, the uncertainties to be compensated for are generated by an exogenous system whose dynamics are either completely unknown or partially unknown. The only requirements on the exogenous system are that its unknown dynamics are bounded by a known function and that its output is bounded. The proposed robust control is based on a nonlinear observer that estimates the uncertainties. It is shown that, under different sets of conditions, local, semiglobal, or global stability of uniform ultimate boundedness or asymptotic stability can be achieved.