Overrepresented subgraph patterns in biological regulatory networks
Network motifs (Milo et al. 2002) are subgraph patterns that occur far more often in real networks than in random graphs with the same degree sequence. The bi-fan (two sources → two targets, all four edges) is the most common 4-node motif in transcription networks. The feedforward loop (FFL, 3-node) acts as a pulse generator or sign-sensitive delay. Z-score = (N_real − ⟨N_rand⟩) / σ_rand.