The identification of mutations at codons 515 or 505 of the MPL gene, particularly in the absence of the JAK2 p.V617F mutation, holds clinical significance for the diagnosis of essential thrombocythemia (ET) or primary myelofibrosis (PMF), especially in patients presenting with relevant symptoms and clinical features [43,44,45,46]. The gene discussed is MPL; the disease is essential thrombocythemia.