  We report on the generation of mid-infrared Kerr frequency combs in
high-finesse CaF$_2$ and MgF$_2$ whispering-gallery mode resonators pumped with
continuous wave room temperature quantum cascade lasers. The combs were
centered at 4.5$\mu$m, the longest wavelength to date. A frequency comb wider
than a half of an octave was demonstrated when approximately 20mW of pump power
was coupled to an MgF2 resonator characterized with quality factor exceeding
10$^8$.
