above 100 MeV is still unassociated [@abdo11]. It is worth noting that the most commonly detected sources in the $\gamma$-ray sky, since the epoch of CGRO, are blazars, one of the most enigmatic classes of Active Galactic Nuclei (AGNs) [e.g., @hartman99]. Within the 2FGL, there are 576 UGSs out of a total number of 1873 sources detected, while among the 1297 associated sources, $\sim$ 1000 have been associated with AGNs [@abdo11; @ackermann11a].

Blazar emission extends over the whole electromagnetic spectrum and is generally interpreted as non-thermal radiation arising from particles accelerated in relativistic jets closely aligned to the line of sight [@blandford78]. They come in two flavors: the BL Lac objects, with featureless optical spectra or only with absorption lines of galactic origin and weak and narrower emission lines, and the Flat Spectrum Radio Quasars, with a optical spectra showing broad emission lines. In the following, we indicate the former as BZBs and the latter as BZQs, respectively, according to the ROMA-BZCAT[^1] nomenclature [@massaro09; @massaro10; @massaro11a].

The first step to improve our knowledge on the origin of the UGSs and of their associations with low-energy counterparts, is to recognize those that could have a blazar within their $\gamma$-ray positional uncertainty 