Fractional variability at different frequencies. All the F_var values are computed with the single observations shown in Fig. 1, with the exception of the F_var values related to Fermi-LAT which were computed with 15-day and 30-day time intervals, and depicted with full circles and open circles, respectively. Open symbols for optical bands indicate the fractional variability after subtracting the host galaxy contribution, as determined in Nilsson et al. (2007). For the X-ray and the VHE γ-ray band, open markers depict the variability after removal of flaring episodes from the light curves as described in the text.