Largely simultaneous broadband SED of Mrk 421 on MJD 55 265. The correspondence between markers and instruments is given in the legend. The full names of the instruments can be found in Table 1. Because of space limitations, R-band instruments other than GASP, GRT, and NMS are denoted with the symbol “++”. Whenever a simultaneous observation is not available, the fluxes from the closest date are reported, and their observation time in MJD is reported next to the instrument name in the legend. The red curve depicts the one-zone SSC model matching the data. The gray circles depict the average SED from the 2009 MW campaign reported in Abdo et al. (2011), which is a good representation of the nonflaring (typical) SED of Mrk 421.