  The role of kink instability in magnetically driven jets is explored through
numerical one-dimensional steady relativistic MHD calculations. The instability
is shown to have enough time to grow and influence the dynamics of
Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic
flux dominated at distances larger than ~1000 r_g because of the rapid
dissipation of Poynting flux. When applied to GRB outflows, the model predicts
more gradual Poynting dissipation and moderately magnetized flow at distances
of ~10^{16} cm where the deceleration of the ejecta due to its interaction with
the external medium is expected. The energy released by the instability can
power the compact ``blazar zone'' emission and the prompt emission of GRB
outflows with high radiative efficiencies.
