ZCL 278 is a cell-permeable inhibitor of Cdc42, a Rho family GTPase. It binds into the surface groove on Cdc42 (Kd = 6.4-11.4 µM), blocking interaction with intersectin (ITSN), a guanine nucleotide exchange factor (GEF). It inhibits Cdc42-mediated microspike formation and neuronal branching in 3T3 fibroblast and neonatal cortical neurons, respectively. ZCL 278 also exhibits potent antiviral activity by inhibiting cellular entry of Junin virus (JUNV) and other enveloped viruses into cells and inhibiting viral replication in mice.