  In contrast to engineering applications, in which the structure of control
laws are designed to satisfy prescribed function requirements, in biology it is
often necessary to infer gene-circuit function from incomplete data on
gene-circuit structure. By using the feed-forward loop as a model system, this
paper introduces a technique for classifying gene-circuit function given
gene-circuit structure. In simulations performed on a comprehensive set of
models that span a broad range of parameter space, some designs are robust,
producing one unique type of functional response regardless of parameter
selection. Other designs may exhibit a variety of functional responses,
depending upon parameter values. We conclude that, although some feed-forward
loop models have designs that lend themselves to unique function inference,
others have designs for which the function type may be uncertain.
