The present invention relates to X-ray crystallography studies of a human Janus Kinase 2 (JAK2) domain. More particularly, it relates to the crystal structure of a JAK2 kinase domain bound to a inhibitor. The invention further relates to the use of the crystal and related structural information to select and screen for compounds that interact with JAK2 and related proteins and to compounds that could be used for the treatment of diseases mediated by inappropriate JAK2 activity.