  We investigate the evolutionary properties of a sample of quasars at 5<z<6.4
using the semi-analytical hierarchical model GAMETE/QSOdust. We find that the
observed properties of these quasars are well reproduced by a common formation
scenario in which stars form according to a standard IMF, via quiescent star
formation and efficient merger-driven bursts, while the central BH grows via
gas accretion and BH-BH mergers. Eventually, a strong AGN driven wind starts to
clear up the ISM of dust and gas, damping the star formation and un-obscuring
the line of sight toward the QSO. In this scenario, all the QSOs hosts have
final stellar masses in the range $(4-6)\times 10^{11} M_{sun}$, a factor 3-30
larger than the upper limits allowed by the observations. We discuss
alternative scenarios to alleviate this apparent tension: the most likely
explanation resides in the large uncertainties that still affect dynamical mass
measurements in these high-z galaxies. In addition, during the transition
between the starburst-dominated and the active QSO phase, we predict that about
40% of the progenitor galaxies can be classified as Sub Millimeter Galaxies,
although their number rapidly decreases with redshift.
