Fig. (1) Domain structure of JAK protein family members. JH: Jak homology domains: functional regions are highlighted and include a FERM (Band4.1 ezrin radixin moesin) domain that mediates cytokine receptor binding, an SH2 (Src homology 2) domain and two kinase domains of which only the second is functional. The V617F mutation found in JAK2 is lies within the enzymatically inactive pseudo kinase domain.