  We present model calculations to the observed spectral variability in the
gamma-ray spectrum of the ultraluminous blazar-type quasar PKS 0528+134. We
argue that the observed appearance of a spectral break between the COMPTEL and
the EGRET energy range can plausibly be explained by a variation of the Doppler
beaming factor in the framework of a relativistic jet model for AGNs, combining
the ERC and the SSC model. This may be an instructive example for the different
dependence of these two spectral components on the Doppler factor, as predicted
in earlier work.
