A molecular switch, triggered by a signal, helps an organism instantly recognise a healthy and potential sex mate. "This mating decision is controlled by a simple chemical switch that converts an incoming pheromone signal into a cellular response," says study author Stephen Michnick, University of Montreal biochemistry professor. "As pheromone signal increases, two enzymes in the cell begin to compete with each other, one adding, the other removing a chemical modification on a protein called Ste5," he continues. He noted that at a critical threshold of pheromone signal, one of the enzymes overwhelms the others' capacity to modify Ste5, triggering a sudden, switch-like cascade of chemical messages to be delivered to the cell to say it's time to mate. The findings were made in collaboration with physicist Peter Swain, McGill University and the University of Edinburgh, and his postdoctoral fellow Vahid Shahrezaei, now a lecturer at Imperial College London, Britain. Research was supported by CIHR and their findings were published in the journal Nature.