DEF6 is a phosphatidylinositol 3,4,5-trisphosphate-dependent guanine nucleotide exchange factor that regulates multiple cellular processes through activation of Rho GTPases including RAC1, RhoA, and CDC42. The protein plays a critical role in immune homeostasis by ensuring proper trafficking and availability of the T-cell regulatory protein CTLA-4 at the T-cell surface 1. Mechanistically, DEF6 interacts with the small GTPase RAB11 to facilitate CTLA-4 vesicle trafficking, and mutations disrupting this interaction lead to reduced functional CTLA-4 availability 1. DEF6 also functions as a negative regulator of osteoclastogenesis by suppressing TNF-α-induced bone resorption through regulation of an endogenous IFN-β-mediated autocrine feedback loop 2. Biallelic mutations in DEF6 cause immunodeficiency 87 with systemic autoimmunity, characterized by impaired T-cell regulation and aberrant CTLA-4 homeostasis 1. Patients with DEF6 deficiency present with immune dysregulation similar to other Tregopathies, and treatment with CTLA-4-Ig (abatacept) has shown therapeutic success 13. The protein's role extends beyond immune regulation, as increased DEF6 expression correlates with metastasis and poor prognosis in osteosarcoma 4.