 Alon and his collaborators, focusing on gene transcription networks and metabolic networks in bacteria and yeast, identified “recurring, significant patterns of interconnections” involving two, three, or four nodes, which they called motifs (Milo et al., 2002: 824). One example, occurring in “hundreds of non-homologous gene systems” (Mangan, Zaslaver, &amp; Alon 2003: 197) in the transcription network of E. coli, is the feedforward loop. In a feedforward loop  (Figure 6),  one unit sends inputs to two other units, the first of which also sends an input to the other, which serves as the final output unit. Using Boolean models, Alon and his colleagues demonstrated that depending on whether the connections are excitatory or inhibitory, such a motif can perform a number of different functions. For example, if all the connections are excitatory and the final output unit is only active when it receives input from both of the other units (the connections to it constitute an AND-gate, as shown on the left in  Figure 6),  then the feedforward loop functions as a persistence detector—the output unit only becomes active when the input to the first unit endures at least until the second unit becomes active. This, as Mangan et al. explain, protects the output (which might serve to start the transcription of a gene) from being generated in response to random noise presented to the input unit. The feedforward loop and Alon’s other motifs are characterized abstractly without specific reference to what entities and interactions correspond to the nodes and edges. Philosophers who have examined this research refer to these organizational patterns as design principles (Green 2015; Green, Levy, &amp; Bechtel 2015; Levy &amp; Bechtel 2013).