  We study metal-insulator transitions between Mott insulators and metals. The
transition mechanism completely different from the original dynamical mean
field theory (DMFT) emerges from a cluster extension of it. A consistent
picture suggests that the quasiparticle weight $Z$ remains nonzero through
metals and suddenly jumps to zero at the transition, while the gap opens
continuously in the insulators. This is in contrast with the original DMFT,
where $Z$ continuously vanishes but the gap opens discontinuously. The present
results arising from electron differentiation in momentum space agree with
recent puzzling bulk-sensitive experiments on CaVO$_3$ and SrVO$_3$.
