text: a cells produce a-factor, a mating pheromone which signals the presence of an a cell to neighbouring α cells. a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the Al Capp cartoon character Shmoo) towards the source of α-factor. Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type.
  These phenotypic differences between a and α cells are due to a different set of genes being actively transcribed and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor (Ste2) which binds to α-factor and triggers signaling within the cell. a cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.