Before revision:  we discuss the behavior of Sir protein recruitment, spreading and stability of silenced regions in commonly-studied mutants (e.g., \emph{sas2}%DIFAUXCMD  ,\emph{dot1}%DIFAUXCMD   ) illuminating the controversial role of Dot1 in the systems biology of yeast silencing.
After revision:  we discuss the behavior of Sir protein recruitment, spreading and stability of silenced regions in commonly-studied mutants (e.g., \emph{sas2}%DIFAUXCMD  \emph{dot1}%DIFAUXCMD  ) illuminating the controversial role of Dot1 in the systems biology of yeast silencing.