Background: Chuvash polycythemia is a hematological disorder present worldwide but endemic to the Chuvash population, a Turkish ethnic group in Russia. The disorder is caused by a homozygous germline mutation (R200W) in the von Hippel Lindau gene (1,2). This mutation impairs binding of pVHL to hypoxia-inducible factor 1-alpha (HIF-1a); lack of this interaction prevents degradation of HIF-1a (2,3). The resultant upregulation of HIF-1a, even in a normal oxygen state, increases the activity of erythropoietin, thereby causing polycythemia (2,3). Affected individuals experience increased rates of arterial and venous thrombosis unrelated to the increased concentration of hemoglobin (4,5). Aims: To determine whether upregulation of HIF-1a in Chuvash polycythemia individuals causes a decreased level of the antithrombotic Protein S. A decreased level of Protein S may explain the increased risks of thrombotic events and vascular abnormalities in the Chuvash population. Methods: Enzyme-linked immunosorbent assays (ELISA) were performed to measure total Protein S concentration in Chuvash and control plasma. Immunoblotting was performed to confirm the ELISA measurements. Additional assays to measure free Protein S and thrombin generation assays with and without Protein S will be performed to determine whether Chuvash plasma has low free Protein S and whether Protein S supplementation will improve the thrombotic phenotypes. Results: Total Protein S concentration measured by ELISA was lower in Chuvash individuals compared with controls. In addition, we have completed immunoblotting of seven Chuvash samples and three control samples. Our results indicated that Chuvash plasma had lower amounts of Protein S compared with the controls. Immunoblotting of additional Chuvash samples is in progress, and we are planning to measure the free Protein S in Chuvash samples. Conclusion: Our preliminary results suggest a decreased level of total Protein S in individuals with Chuvash polycythemia. Because an increased hemoglobin concentration does not increase the rates of thromboembolic events in this population, our finding of a reduced amount of anticoagulant Protein S may explain the hypercoagulability that Chuvash individuals experience. References  Gordeuk, V.R., et al., Chuvash polycythemia VHLR200W mutation is associated with down-regulation of hepcidin expression. Blood, 2011. 118(19): p. 5278-82. Gordeuk, V.R., et al., Congenital disorder of oxygen sensing: association of the homozygous Chuvash polycythemia VHL mutation with thrombosis and vascular abnormalities but not tumors. Blood, 2004. 103(10): p. 3924-32. Gordeuk, V.R. and J.T. Prchal, Vascular complications in Chuvash polycythemia. Semin Thromb Hemost, 2006. 32(3): p. 289-94. Sergueeva, A., et al., Prospective study of thrombosis and thrombospondin-1 expression in Chuvash polycythemia. Haematologica, 2017. 102(5): p. e166-e169. Gordeuk, V.R., N.S. Key, and J.T. Prchal, Re-evaluation of hematocrit as a determinant of thrombotic risk in erythrocytosis. Haematologica, 2019. 104(4): p. 653-658. Disclosures Gordeuk: Modus Therapeutics: Consultancy; Novartis: Research Funding; Incyte: Research Funding; Emmaus: Consultancy, Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; CSL Behring: Consultancy.