The mating behavior of yeast depends on signaling peptides termed pheromones that bind to pheromone GPCRS (see figure below). When the -factor pheromone binds to a wild-type yeast cell, it blocks cell-cycle progression, arresting proliferation until a mating partner is found. Yeast mutants with defects in one or more of the components of the G protein have characteristic phenotypes in the absence and in the presence of the -factor pheromone (see table). Strains with defects in any of these genes cannot undergo the mating response and are therefore termed sterile.