The conserved Rho-type GTPase Cdc42p is a key regulator of signal trans-duction and polarity in eukaryotic cells. In the yeastSaccharomyces cerevi-siae, Cdc42p promotes polarized growth through the p21-activated kinases Ste20p and Cla4p. Previously, we demonstrated that Ste20p forms a com-plex with Erg4p, Cbr1p and Ncp1p, which all catalyze important steps in sterol biosynthesis.