Before mating, a yeast cell must detect a partner cell that is close enough and expresses sufficiently large amounts of a sex pheromone. The mating decision is an all-or-none, switch-like response to pheromone concentration. It is now shown that this decision involves the competition of one kinase and one phosphatase enzyme for multiple phosphorylation sites on a 'scaffold' protein. The results should prompt a re-evaluation of the role of related signalling molecules that have been implicated in cancer