Finding the brakes on the Toxoplasma life cycle.

Toxoplasma gondii motility is an all-or-nothing response. Claywell et al. identify negative-feedback loops in cyclic-nucleotide signaling that allow parasites to turn off motility and commit to intracellular replication. Feedback mechanisms and bistability are useful frameworks for describing, modeling, and testing T. gondii motility signaling pathways.