Tsz-Leung To and Narendra Maheshri at MIT describe positive-feedback loops in gene regulatory networks which may exhibit bimodal responses in Science this week. Using a synthetic system in budding yeast, To and Maheshri show that “a promoter with multiple transcription factor binding sites can induce a steady-state bimodal response without cooperative binding of the TF.” The researcher’s deterministic models did not predict bistability, however, which is a finding contrary to common thought.