Evolution with time of some physical parameters of Mrk 421 in the core region at 15 GHz (upper frame), 24 GHz (middle frame), and 43 GHz (lower frame). For each frame we report from the first to the fourth panel the light curves for the total intensity, the polarized flux density, the fractional polarization, and the EVPA values. Triangles in the lower frame (first panel) represent the VLBA 43 GHz observations of Mrk 421 provided by the VLBA-BU-BLAZAR program.