  While the best tracer of the molecular component and its dynamics in galaxies
is the CO molecule, which excitation is revealed by its isotopic and
(2-1)/(1-0) ratios, the denser gas is revealed by molecules such as HCN, HNC,
HCO+ or CN, which are now widely used to probe star formation regions, or to
quantify the impact of the nuclear activity on the interstellar medium.
  This paper reviews recent observations in nearby galaxies, where these
molecular line ratios serve as diagnostic tools of the physical conditions of
the gas and also of its chemical properties. Those differ significantly
according to the proximity of an AGN or of a starburst. The origin of the
differences is not yet well known and could be due to different densities,
temperatures, chemical abundances or non-collisional excitation of the gas
(e.g. Aalto et al 2007, Krips et al 2007).
  HCN or HNC line enhancements can be caused not only by higher gas
densities/temperatures, but also UV/X-ray radiation, and global IR pumping. The
chemistry can be dominated by PDR regions near a starburst, or X-ray dominated
in a molecular torus surrounding an AGN (XDR regions). The molecular line
ratios expected in those regions vary according to the different models
(Meijerink et al. 2007).
