The JAK-STAT signal-transduction pathway is utilized by a wide range of cytokines to regulate gene expression. Cytokines activate members of the JAK family o f protein tyrosine kinases, which in turn activate, by tyrosine phosphorylation, one or more STAT transcription factor family members. Activated STATs form dimers, translocate to the nucleus and bind to response elements to induce transcription. Recent findings are beginning to connect JAKs and STATs with other signalling pathways: JAKs may phosphorylate and activate signalling proteins other than STATs, and STATs can be phosphorylated by non-]AK tyrosine kinases. STAT activity can also be modulated by serine phosphorylation.