  Monotone systems constitute one of the most important classes of dynamical
systems used in mathematical biology modeling.
  The objective of this paper is to extend the notion of monotonicity to
systems with inputs and outputs, a necessary first step in trying to understand
interconnections, especially including feedback loops, built up out of monotone
components.
  Basic definitions and theorems are provided, as well as an application to the
study of a model of one of the cell's most important subsystems.
