Neurofibromatosis type 1 (NF1) is a common cancer predisposition syndrome caused by mutations in the tumor suppressor gene. encodes neurofibromin, a GTPase-activating protein for RAS proto-oncogene GTPase (RAS). Plexiform neurofibromas are a hallmark of NF1 and result from loss of heterozygosity of in Schwann cells, leading to constitutively activated p21RAS. Given the inability to target p21RAS directly, here we performed an shRNA library screen of all human kinases and Rho-GTPases in a patient-derived Schwann cell line to identify novel therapeutic targets to disrupt PN formation and progression. Rho family members, including Rac family small GTPase 1 (RAC1), were identified as candidates. Corroborating these findings, we observed that shRNA-mediated knockdown of RAC1 reduces cell proliferation and phosphorylation of extracellular signal-regulated kinase (ERK) in Schwann cells. Genetically engineered mice, which develop multiple PNs, also exhibited increased RAC1-GTP and phospho-ERK levels compared with littermates. Notably, mice in which both and loci were disrupted ( ) were completely free of tumors and had normal phospho-ERK activity compared with ; mice. We conclude that the RAC1-GTPase is a key downstream node of RAS and that genetic disruption of the allele completely prevents PN tumor formation in mice.