I will present a new technique for extending frequency combs to the highly desirable mid-IR ?molecular fingerprint? spectral range. The technique is based on subharmonic optical parametric oscillation (OPO) in quasi-phase-matched GaAs crystal. This process can be considered as a reverse of the second harmonic generation: the frequency comb of a pump laser (Tm-fiber 2-?m laser or Cr:ZnS 2.4-?m laser) is transposed to half of its central frequency and simultaneously spectrally augmented to more than an octave, thanks to the enormous gain bandwidth near OPO degeneracy and cross-coupling between the laser and the OPO frequency comb lines. With this source we demonstrate massively-parallel trace detection of molecular mixtures and achieve part-per-billion sensitivity levels. Our present focus is both biomedical and security applications of mid-IR spectral combs.