The two kinds of systems we consider, chemical rate equations and S-systems, have previously been used for identification of ODE models as indicated by references in Table 1. In addition to these kinds of systems, other fixed forms have been considered in the literature. One example is Generalized Mass Action (GMA) models (Tucker et al., 2007; Voit, 2000), and another is linear dynamical models with non-linear transfer functions of sigmoid type (D'haeseleer, 2000; Nelander et al., 2008; Wahde and Hertz, 2000). However, at this stage, we have chosen to focus on the more frequently used S-systems as one representative fixed form system.