  We show that when Bjorken $x_B$ is large, the leading order twist-four
contributions to the structure functions can be expressed in terms of the
derivatives of the normal twist-two parton distributions. Our analytical
results are not only consistent with the parameterized power corrections
extracted from the data, but also predict the flavor dependence of the power
corrections. We also discuss the impact of our analytical results on the
extraction and the flavor separation of the parton distributions at large $x$.
