JAK2 and myeloproliferative disorder: In 2005, the somatic mutation in the Janus Kinase 2 (JAK2) gene was described; their replacement of the amino acid valine with the amino acid phenylalanine at codon 617 (V617T) involves increased activity of the JAK2 tyrosine kinase protein and is characteristic of pathological clone in myeloproliferative diseases.