Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): In Saccharomyces cerevisiae, the functions of the Rho-type GTPase Cdc42p that are required for cell division and cellular development can be uncoupled by single amino acid substitutions. Pseudohyphal development of S. cerevisiae, but not cell division, is blocked by exchange of any of the green residues. Exchanges of red residues enhance pseudohyphal development without affecting cell division. Switch I and switch II domains of Cdc42p are yellow, and the Rho Insert domain is purple. (See related article on page 235.)