In Saccharomyces cerevisiae, Ste50 functions in cell signalling between the activated G protein and the mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) Ste11. ScSte50 is an essential component of three MAPK-mediated signalling pathways, which control the mating response, invasive/filamentous growth and osmotolerance (HOG pathway), respectively. ScSte50 signalling may also contribute to cell wall integrity in vegetative cells. The protein contains a sterile alpha motif (SAM) and a putative Ras-associated domain (RAD), which are essential for signal transduction. Ste50 and Ste11 interact constitutively via their SAM regions. Ste50 interacts weakly and probably transiently with the pheromone receptor-bound heterotrimeric G protein G(alpha beta gamma), and with the small G proteins Cdc42, Ras1 and Ras2. It is specifically the RAD region of Ste50 that mediates the interactions with Cdc42 and Ras. Homologues of ScSTE50 are also found in other fungi, like S. kluyveri, Hansenula polymorpha, Candida albicans and Neurospora crassa. In this review, the role of Ste50 as an adaptor that links the G protein-associated Cdc42-Ste20 kinase complex to the effector kinase Ste11 and thus modulates signal transduction, especially in the pheromone-response pathway of S. cerevisiae, is discussed.