Most of the receptors for soluble factors functioning in immune and hematopoietic systems belong to the cytokine receptor family. These receptors often share common signal transducing receptor components with other members of the same family. Such receptors and signal transducers possess no intrinsic tyrosine kinase domain but have recently been found to be associated with members of a JAK family of cytoplasmic tyrosine kinases. The JAK kinases become activated after ligand-induced dimerization of the receptor components. This activation appears to link the cell surface receptors to the nuclear genes through tyrosine phosphorylation and activation of latent cytoplasmic transcription factors called signal transducers and activators of transcription (STATs).