Network Motif — Feedforward Loop Dynamics

The FFL: how gene networks achieve pulse generation, delay, and sign-sensitive filtering

Feedforward Loops

Most common regulatory motif

In a FFL, transcription factor X activates Y; both X and Y regulate output Z. The sign of each edge determines function.

Ẏ = β_Y·f(X) − α_Y·Y
Ż = β_Z·g(X,Y) − α_Z·Z

Coherent FFL (C1): X→Y→Z, X→Z (all +). Acts as a persistence detector — delays activation, fast off.

Incoherent FFL (I1): X→Y→Z but X⊣Z. Generates a pulse, acts as fold-change detector.