ARHGEF18 is a guanine nucleotide exchange factor (GEF) that primarily activates RhoA GTPase to regulate actin cytoskeleton dynamics and cell polarity 1. The protein exists in multiple isoforms, including the canonical p114 form and leukocyte-specific LOCGEF variants, with distinct tissue-specific expression patterns 2. Beyond RhoA activation, ARHGEF18 also acts as a GEF for RAC1, promoting reactive oxygen species production [UniProt annotation]. ARHGEF18 is emerging as a critical regulator of vascular and epithelial barrier function. Recent evidence demonstrates that ARHGEF18 is phosphorylation-dependent and flow-responsive in endothelial cells, where it maintains tight junctions and controls vascular permeability through RhoA-mediated signaling 3. During epithelial migration, p114 ARHGEF18 accumulates at adherens junctions to stabilize E-cadherin and promote collective cell movement 4. ARHGEF18 also participates in early mitochondrial fission through activation by SEPTIN9 5. In development, ARHGEF18 maintains neuroepithelial apicobasal polarity and controls proliferation via RhoA-Rock2 signaling 1. Clinically, ARHGEF18 dysregulation associates with retinitis pigmentosa 78 and pulmonary arterial hypertension 6, and machine learning studies identify ARHGEF18 as a diagnostic biomarker for sepsis and septic shock 78.