  Precise measurements of polarized electro-production and Drell-Yan in the
deep inelastic limit will soon provide first information on the higher twist
parton distributions $g_T(x)$ and $h_L(x)$. Sum rules for higher-twist
structure functions are only valid provided the corresponding Compton
amplitudes satisfy un-subtracted dispersion relations. Subtracted dispersion
relations have to be used when the (real part of the) forward scattering
amplitudes does not fall off rapidly enough for $\nu \rightarrow \infty$ (fixed
$Q^2$). Formally, such subtractions lead to $\delta$-functions at the origin in
the parton distributions, which are not accessible to experiment, and the
integral over the data fails to satisfy the sum rule. The $\delta$-functions in
the parton distributions can be identified with the zero-modes that appear in
light-front quantization. An explicit {\it infinite momentum boost} identifies
these soft quark modes with low momentum contributions to the self-energy.
