The single Ras homologue (Ras1) of S. pombe regulates two distinct processes: (1) Signal transduction through a MAP kinase-like protein kinase cascade in response to mating pheromones. In this pathway Ras1 interacts with the protein kinase Byr2 and leads to its activation in conjunction with a signal from the receptor-coupled, heterotrimeric G protein. (2) Polarized cell growth both during the cell cycle and during directed cell extension towards a mating partner. Ras1 interacts with Ral1/Scd1, a putative guanine-nucleotide-exchange factor, which could activate Cdc42, Rho-like GTP-binding protein. Cdc42 may regulate the dynamics of the actin cytoskeleton.