The broad, long-term objectives of this work are to use the genetically tractable system of C elegans to identify and define the role of the small molecular weight GTP binding protein cdc42 in C. elegans embryonic development. This information may offer significant insight into the role and function of cdc42 in mammalian cells, the study of which is often hampered by the limits of the system. cdc42 is known to participate in signal transduction, cell cycle control and the establishment of cell polarity. An activated mutant of cdc42 transforms Swiss 3T3 cell. The ability of cdc42 to cause uncontrolled cell growth as well as its role in several processes controlling cell function make understanding cdc42 function an important health related question. The C. elegans system offers an opportunity for a molecular and genetic investigation into cdc42 function in an organism that undergoes metazoan development. Early C elegans development depends on a number of asymmetric cell divisions that require the function of several identified genes, par-1 through par-6 and a member of the Wnt family mom-2. Our results, using the RNA interference technique, suggest that cdc42 is also required to establish cell polarity in the early embryo. To begin to investigate the role of cdc42 in C. elegans development and its interaction with other polarity determining pathways, we propose to do the following: Specific Aim 1. Generate mutant lines of C. elegans with altered activity in the cdc42 gene. Specific Aim 2. Identify and characterize a possible role for cdc42 in the generation of asymmetry, polarity, blastomere fate determination, membrane restructuring such as cytokinesis or other events during embryonic development in C. elegans. Specific Aim 3. Identify proteins that interact with GTP-cdc42 using a two hybrid screen for interacting peptides.