  We have developed a model for gamma ray emission in jets of active galactic
nuclei in which particle acceleration takes place at a shock in the
relativistic jet plasma due to a massive star in the central region of the host
galaxy moving through the jet. The gamma rays are produced in a pair-Compton
cascade in the radiation field of the star initiated by accelerated electrons.
Our model may account for the observed GeV to TeV gamma ray spectrum and
variability of Markarian 421 and other blazars detected by the EGRET instrument
on the Compton Gamma Ray Observatory.
