  Medium modification of parton fragmentation functions induced by multiple
scattering and gluon bremsstrahlung is shown to describe the recent HERMES data
in deeply inelastic scattering (DIS) very well, providing the first evidence of
$A^{2/3}$-dependence of the modification. The energy loss is found to be
$<dE/dL> \approx 0.5$ GeV/fm for a 10-GeV quark in a $Au$ nucleus. Including
the effect of expansion, analysis of the $\pi^0$ spectra in central $Au+Au$
collisions at $\sqrt{s}=130$ GeV yields an averaged energy loss equivalent to
$<dE/dL> \approx 7.3$ GeV/fm in a static medium. Predictions for central
$Au+Au$ collisions at $\sqrt{s}=200$ GeV are also given.
