Jak-Stat Pathways. Jak-Stat pathways entail the receptor Janus kinase (Jak) and the signal transducer and activator of transcription (Stat). Stat molecules are inactive as monomers, and their activation involves phosphorylation and dimerization. The binding of the ligand (here the hormone Epo) to the receptor (EpoR) results in phosphorylation of Jak and of the cytoplasmatic domain of EpoR. Monomeric Stat5 is recruited to the phosphorylated and thereby activated receptor, EpoRA. (1) Upon receptor recruitment, monomeric Stat5 is tyrosine-phosphorylated. It dimerizes (2) and migrates to the nucleus (3), where it binds to the promoter of target genes. After dissociation (4), it is dephosphorylated and exported to the cytoplasm.