Ribbon diagram of a three-dimensional model of the structure of S. cerevisiae Cdc42, generated with Swiss-Model83, based on alignment of the amino acid sequence of yeast Cdc42 with human Cdc42 (80% identity) and the crystal structures of GTP-bound Cdc42Hs (Protein Data Bank entries 2NGR, 1GRN, 1AN0 and 1A4R), drawn with MOLSCRIPT84. Switch I and II elements are shown in yellow. Side-chains of Ile46, Ser71 and Glu100 are indicated as ball-and-stick representations in turquoise. The nucleotide has been omitted for clarity. Cdc42 is a protein required for cell viability; mutations of the residues indicated do not cripple its essential function(s), but do eliminate its ability to stimulate two different MAPK pathways (Ser71, pheromone response pathway; Ile46 and Glu100, filamentous growth pathway).