JAK2 and thrombocytosis disease: Thrombocytosis and the increased hematocrit, which are natural characteristics of these disorders, also cause thrombosis in the venous systems.12 A substantial number of patients with SVT, especially portal vein thrombosis (PVT) were found to be carriers of mutation in Janus kinase 2 (JAK2) in the absence of overt signs of chronic myeloproliferative disease.13