Of the 269 patients with ET, 179 (66·5%) carried JAK2 V617F, 48 (17·8%) a CALR exon 9 indel, 9 (3·4%) an MPL mutation, and 33 (12·3%) had nonmutated JAK2, CALR, and MPL (ie, triple-negative patients). This evidence concerns the gene JAK2 and essential thrombocythemia.