ABR (ABR activator of RhoGEF and GTPase) encodes a bifunctional regulatory protein with opposing activities toward small GTP-binding proteins. The protein contains a C-terminal GTPase-activating protein (GAP) domain that stimulates GTP hydrolysis by RAC1, RAC2, and CDC42, leading to downregulation of their active GTP-bound forms 1. Additionally, ABR possesses a central Dbl homology (DH) domain that functions as a guanine nucleotide exchange factor (GEF), promoting the conversion of CDC42, RHOA, and RAC1 from GDP-bound to GTP-bound forms 1. This unique dual functionality allows ABR to both activate and inactivate small GTPases depending on cellular context. ABR shows differential tissue expression with highest levels in brain and exhibits genomic variability due to tandem repeat regions in introns 2 1. The protein plays important roles in cellular processes including phagocytosis, where ABR knockdown reduces phagocytic activity by 40% in trabecular meshwork cells 3. ABR is downregulated by dexamethasone treatment, which may contribute to impaired phagocytosis in glucocorticoid-induced ocular hypertension 3. Unlike the related BCR protein, ABR lacks a serine/threonine kinase domain, suggesting distinct cellular functions despite overlapping GAP activities 1.