Dameshek’s concept of MPN was genetically ratified in 2005 by the seminal discovery, across these three clincopathologic entities, of a JAK2 gain of-function mutation (JAK2V617F; a G to T somatic mutation at nucleotide 1849, in exon 14, resulting in the substitution of valine to phenylalanine at codon 617) [4–7]. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.