Primary myelofibrosis (PMF) is a Philadelphia chromosome-negative (Ph-neg) chronic myeloproliferative neoplasm (MPN) characterized by the following hallmarks: bone marrow (BM) fibrosis, myeloid metaplasia, splenomegaly, increased frequency of circulating CD34+ hematopoietic progenitor cells (HPCs), and a V617F mutation of the JAK2 gene in the hematopoietic lineage encountered in 63% of the patients [1], [2]. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.