Radiative energy loss of a highly virtual quark originating from a deep-inelastic electron scattering plays a crucial role in production of leading hadrons off nuclei. The density of energy loss for gluon radiation turns out to be time- and energy--dependent in inclusive hadron production. Important phenomena involved are Sudakov's suppression of no radiation of that part of gluon spectrum, which is forbidden by energy conservation, and color transparency, which suppresses the final state interaction of the produced colorless wave packet. We model the soft part of hadronization, which usually is supposed to be due to the color strings, using also the gluon radiation and nicely reproduce the string parameters. Our parameter-free calculations provide a good agreement with available data on $z_h$-dependence of the quark fragmentation function in vacuum, as well as $\nu$-, $z_h$- and $Q^2$-dependence of nuclear effects. We come to the conclusion that the energy range of ELFE - HERMES is most sensitive to the underlying dynamics of hadronization provided that nuclear targets are used.