  A complete list of the so-called Lorentz invariance relations between parton
distribution functions is given and some of their consequences are discussed,
such as the Burkhardt-Cottingham sum rule. The violation of these relations is
considered in a model independent way. It is shown that several Lorentz
invariance relations are not violated in a generalized Wandzura-Wilczek
approximation, indicating that numerically their violation may be small.
