  Blazars can be classified on the basis of their overall Spectral Energy
Distribution (SED). BL Lac objects are usually divided in LBL or HBL (Low or
High energy peaked BL Lacs), according to the peak frequency of the synchrotron
emission, if in the optical or UV-soft-X band respectively. FSRQs instead are
characterized by synchrotron peaks mainly at IR-optical frequencies, similarly
to LBLs. Here we report on recent BeppoSAX observations which are unveiling the
high energy branch of the range of synchrotron peak frequencies. Four new
"extreme" HBLs have been discovered, one of which (1ES 1426+428) peaks near or
above 100 keV, in a quiescent state.
