We stratified patients with ET according to their somatic driver mutation into patients heterozygous for JAK2 mutation (JAK2het; that is, JAK2 mutation without chromosome 9 aberrations; n = 308), patients with CALR and/or MPL mutations (n = 169), and patients negative for JAK2, MPL and CALR mutations (triple-negative (TN) patients; n = 81). The gene discussed is MPL; the disease is essential thrombocythemia.