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Preface In 1955 we edited three volumes of Methods in Enzymology (255, 256, 257) dedicated to small GTPases. Since then this field has exploded, and these monomeric, regulatory proteins are now firmly established as a common focus of interest in a wide variety of research areas including cell and developmental biology, immunology, neurobiology, and, more recently, microbiology. After talking with colleagues, it became apparent that all three volumes needed to be significantly updated. We have, therefore, attempted to identify the major new areas and themes that have emerged. This volume covers the Rho GTPase family. These proteins are key regulators of the actin cytoskeleton, and since the last volume on the subject there has been significant progress in identifying and characterizing the biochemical pathways associated with the three best characterized members of this family, Rho, Rac, and Cdc42. In the past five years, interest has also widened to a much broader community, as it has become clear that Rho GTPases also participate in the regulation of many other signaling pathways, notably activation of the JNK and p38 MAP kinase pathways and of transcription factors such as SRF and NF-KB. This ability to coordinately regulate changes in the actin cytoskeleton with changes in gene transcription and other associated activities appears to be conserved from yeast to mammals. When the last volumes were published, the large diversity of both downstream targets and upstream guanine nucleotide exchange factors that interact with Rho GTPases was not fully appreciated. Not surprisingly, therefore, these figure more prominantly this time around. Also, although it was thought likely that Rho GTPases might participate in many processes dependent on the organization of filamentous actin, it has now been directly shown that these proteins control cell movement, phagocytosis, growth cone guidance, and cytokinesi