These mutations often constitutionally activate the JAK/STAT signaling pathway [2,3,4]; while other MPNs have different driver mutations: chronic neutrophilic leukemia harbors a mutation of the CSF3 receptor (CSF3R), which locates on chromosome 1p34.3 and encodes the transmembrane receptor for granulocyte colony stimulating factor (G-CSF; CSF3) [5]; chronic eosinophilic leukemia, not otherwise specified (NOS), is defined, among other criteria, as not being a myeloid/lymphoid neoplasm associated with eosinophilia and rearrangement of PDGFRA, PDGFRB, or FGFR1, or with PCM1-JAK2. The gene discussed is PCM1; the disease is Chronic Eosinophilic Leukemia, Not Otherwise Specified.