Figure 1. Defining a multi‐functional circuit.

• A
  In order to achieve multiple functions it has been proposed that circuits can be structurally modular, i.e they allocate distinct highly interconnected and non‐overlapping sets of genes to each individual function (Di Ferdinando et al, 2001; Solé & Valverde, 2008; Clune et al, 2013; Ellefsen et al, 2015). In this scenario modules do not overlap.
• B, C
  Partial module overlap (Panovska‐Griffiths et al, 2013; Sorrells et al, 2015). The AC/DC circuit is able to alternate between distinct behaviors upon a change in the strength of its gene interactions. This circuit is formed by the superimposition of two distinct modules, a mutual inhibition motif and a repressilator motif, that combine under the same topology. As the strength of specific repressive interactions is adjusted, the AC/DC circuit switches between two distinct dynamical behaviors, that is, a bistable switch or oscillatory behavior, using the mutual inhibition and repressilator circuits, respectively.
• D
  Hypothetical scenario describing a complete module overlap: The same collection of interacting genes is essential to both functions.