The Rho family of GTPases regulates diverse signal transduction pathways. One member, the cell division control protein 42 homolog (Cdc42), is involved in filopodia function, cell polarity, cell cycling, and other processes. CASIN is an indole-based small molecule that inhibits activation of the Cdc42 GTPase (IC50 = 2 µM for inhibiting actin assembly in PIP2-stimulated actin polymerization assays). This compound has been used to explore the role of Cdc42 in the division, survival, and differentiation of hematopoietic stem cells.