  We propose a stochastic model for intracellular transport processes
associated with the activity of molecular motors. This out-of-equilibrium
model, based on a generalized Langevin equation, considers a particle immersed
in a viscoelastic environment and simultaneously driven by an external random
force that models the motors activity. An analytical expression for the mean
square displacement is derived, which exhibits a subdiffusive to superdiffusive
transition. We show that the experimentally accessible statistical properties
of the diffusive particle motion can be reproduced by this model.
