  The discovery of rapid and simultaneous TeV/optical (UV, X-ray) variability
provides strong evidence against the interaction between a thermal radiation
field produced by an accretion flow and relativistic electrons in a jet as the
origin of the gamma-rays in BL Lacertae objects. Synchrotron-self-Compton
scattering (SSC) or proton-initiated cascades (PIC) remain viable mechanisms.
The mechanisms predict very different magnetic field strengths and upper
cut-off energies in the gamma-ray sources, i.e. the field strengths in SSC
models are more than two orders of magnitude lower than in PIC models and the
PIC spectra do not cut-off at TeV.
