Extracellular pheromones bind to cell surface receptors and stimulate the activation of the kinase STE20. This leads to the activation of the MAPKKK STE11 and the subsequent members of this MAP kinase cascade, STE7, FUS3 (also called DAC2) and KSS1 (1). These MAP kinases activate STE12 and FAR1, which effect transcriptional and morphological changes necessary for mating (2,3). CDC42, a small GTP-binding protein, is thought to activate STE20. CDC42 also plays a role in the polarization of budding (4). CLA4, a homolog of STE20, interacts with CDC42 and is also involved in budding and cytokinesis (5,6). CDC11 is also required for cytokinesis and is present at the bud neck during cell division (7). Ypk1 and Ypk2 are functionally overlapping protein kinases that are critical to the normal proliferation of yeast cells (8).