  We present the results of a recently conducted mm-experiment at the IRAM 30m
telescope. We searched in two QSO hosts for the HCN(J=1-0) line emission
tracing the dense regions of gas. Our goal is to probe the HCN-CO line ratio in
QSO hosts for the first time and to compare the results with recent findings of
quiescent galaxies. These findings indicate a strong correlation between the
star formation efficiency and the HCN-CO line ratio over several orders of
magnitudes in infrared luminosities.
