A cell-permeable, reversible, and selective Rac1 inhibitor; inhibiting Rac1 activation by the Rac-specific guanine nucleotide exchange factors (GEFs) TrioN and Tiam 1 (IC₅₀ = 50 μM) without affecting the closely related GTPases, Cdc42, and RhoA activation. NSC 23766 has been used to investigate the role of Rac1 in such diverse cellular functions as stem cell mobilization, epithelial cell migration, angiogenesis, leukemia cell migration and growth, etc.