  We present a sample of 17 high-redshift (3.5<z<5.2) QSO candidates in the 320
sq.arcmin area of the Great Observatories Origins Deep Survey, selected in the
magnitude range 22.45<z_{850}<25.25 using deep imaging with the Advanced Camera
for Surveys onboard the Hubble Space Telescope and the Advanced CCD Imaging
Spectrometer onboard the Chandra X-ray Observatory. On the basis of seven
spectroscopic and ten photometric redshifts we estimate that the final sample
will contain between two and four QSOs with 4<z<5.2. A dearth of high-redshift,
moderate-luminosity (M_{145}=~-23) QSOs is observed with respect to predictions
based on a) the extrapolation of the z~2.7 luminosity function (LF), according
to a pure luminosity evolution calibrated by the results of the Sloan Digital
Sky Survey; and b) a constant universal efficiency in the formation of
super-massive black holes (SMBHs) in dark-matter halos. Evidence is gathered in
favor of a density evolution of the LF at high redshift and of a suppression of
the formation or feeding of SMBHs in low-mass halos.
