The authors concluded about the need to include the search for a mutation in the PIEZO1 gene in children and young adults with idiopathic erythrocytosis and more generally when erythrocytosis appears before the age of 50 years, especially when associated with iron overload, splenomegaly, elevated mean corpuscular haemoglobin concentration, increased reticulocytes, haemolysis or decreased P50 (after excluding a variant of Hb with high oxygen affinity). The gene discussed is PIEZO1; the disease is Tangier disease.