  We discuss a higher-twist factorization procedure of the hadron tensor in
deep inelastic scattering, proposed by Qiu, and extend it to include quark mass
terms. Its property that the hard scattering parts are electromagnetic gauge
invariant separately, is manifestly preserved. Using an auxiliary parton, a
so-called spurion, to generate the quark mass terms, a simple parton model
interpretation is also retained.
