Ciliates communicate via diffusible signaling pheromones that have been identified in association with genetic mechanisms of mating types, either binary or multiple, that control the ciliate-specific sexual phenomenon of conjugation. Except for a glycoprotein and a tryptophan-related molecule acting as pheromones in Blepharisma, the other pheromones are cysteine-rich proteins varying in length among Euplotes species from 32 to 109 amino acid residues. These proteins are all structurally homologous sharing a common three-helix bundle fold, which allows them to bind cells in competition with one another in close analogy with the families of protein growth factors and cytokines of multicellular organisms. Although identified in the extracellular environment for their mating-induction activity, pheromones act also, and presumably primarily, as autocrine growth factors that bind to, and promote the reproduction of the same cells from which they are constitutively released.