In 19.61% (n = 10) of ET patients with major thrombosis, hereditary thrombophilia mutations were detected: 50% (n = 5) of the patients had mutations in the MTHFR gene, 20% (n = 2) of patients had mutations in the PAI-1 gene, 10% (n = 1) of patients had factor V Leiden thrombophilia, 10% (n = 1) of patients had factor II thrombophilia and 10% (n = 1) of patients had factor XIII thrombophilia. The gene discussed is SERPINE1; the disease is thrombophilia.