Three types of molecular markers are associated with Phi-negative MPNs: activating mutations in the JAK2 gene (JAK2-V617F being the most frequent mutation, present in all subtypes of MPNs); activating mutations in the MPL gene (MPL-W515L/K mostly); and alterations of CALR, the gene coding calreticulin (CALR), detected in ET and in PMF [3–11]. The gene discussed is MPL; the disease is essential thrombocythemia.