Janus kinase 2,commanly called JAK2,is a human protein that has been implicated in signaling by members of the type II cytokine receptor family, the GM-CSF receptor family, the gp130 receptor family, and the single chain receptors. Exactly, JAK2 kinase is a member of a family of tyrosine kinases involved in cytokine receptor signaling. The JAK2 gene is mapped to 9p24.1. The JAK2 gene encodes a 1,132-amino acid protein that shares 95% sequence similarity to rat and pig Jak2.JAK2 is constitutively associated with the prolactin receptor and that it is activated and tyrosine phosphorylated upon PRL binding to the PRL receptor. JAK2, and more specifically just its intact N-terminal domain, binds to EPOR in the endoplasmic reticulum and promotes its cell surface expression.The human JAK2 is present in the nucleus of hematopoietic cells and directly phosphorylates tyr41 on histone H3.