  The lysogeny maintenance switch in phage lambda is one of the simplest
examples on the molecular level of computation, command and control in a living
system. If, following infection of the bacterium E. coli, the virus enters the
lysogenic pathway, it represses its developmental functions, and integrates its
DNA into the host chromosome. In this state the prophage may be passively
replicated for many generations of E. coli. In fact, this repressed state is
intrinsically more stable than the gene encoding the repressor. We develop a
mathematical formalism to predict the stability of such epigenetic states from
affinities of the molecular components. We apply the model to the behavior of
recently published mutants at the right operator complex of lambda, and find
that the reported stability indicates that the current view of the switch is
incomplete. The approach described here should be generally applicable to the
stability of expressed states
