This study is a nationwide study including all 66 Danish patients who fulfilled the clinical criteria of JPS and/or had a PV in SMAD4 or BMPR1A. We found that almost 90% of patients with a clinical diagnosis of JPS had a PV in either BMPR1A or SMAD4. Endoscopy had been performed in 52 patients, and 21 (40%) of these did not fulfill the Jass criteria. This evidence concerns the gene SMAD4 and acquired polycythemia vera.