of comparison (ROC) for a generic [[*Fermi*]{}]{} source, with respect to the searching region (SR) centered on the position reported in the 2FGL catalog. The radius of both regions is $R=\theta_{999}$ and they are separated by 2.5$\sqrt{2}$ deg of distance.[]{data-label="fig:roi"}](./roi.pdf){height="6.8cm" width="9.5cm"}

For each unidentified $\gamma$-ray source we defined the [*searching region*]{} (SR) corresponding to a circular region of radius $R$=$\theta_{999}$, centered on the position given in the 2FGL, where $\theta_{999}$ is the major axis of the elliptical source location region corresponding to the 99.9% level of confidence. In addition, we also considered a [*region of comparison*]{} (ROC) defined as a circular region of the same radius $R$, but lying at 2.5$\sqrt{2}$ deg angular distance from the 2FGL position. A schematic view of the locations of the SR and the ROC is shown in Figure \[fig:roi\].

Successively, for every unassociated gamma-ray source in the 2FGL catalog, we ranked all the [[*WISE*]{}]{} sources within its SR on the basis of the classification described above and we selected as $\gamma$-ray blazar candidates the positionally closest sources with the highest class. In our analysis we considered only sources of the [[*WISE*]{}]{} preliminary catalog detected in all the four [[*WISE*]{}]{} bands, without any upper limit.

