  Let $X$ and $Y$ be two smooth projective varieties such that there is a fully
faithful exact functor from $D^b(\mathrm{Coh}(X))$ to $D^b(\mathrm{Coh}(Y))$.
We show that $X$ and $Y$ are birational equivalent if the functor maps one
skyscraper sheaf to a skyscraper sheaf. Further assuming that $X$ and $Y$ are
of the same dimension, we show that if $X$ has ample canonical bundle and
$H^0(X ,K_X)\neq 0$, or if $X$ is a K3 surface with Picard number one, then $Y$
is birational to a Fourier--Mukai partner of $X$.
