The PRL-R is a member of the type 1 cytokine receptor family,^[90]and its nearest relative is the GH receptor. Several hematopoietic cytokine receptors, such as those for erythropoietin, most interleukins, and granulocyte-macrophage colony-stimulating factor, are also very similar to PRL and GH receptors. Other receptors, such as those for the interferons, are members of a broader superfamily of proteins that includes cell adhesion proteins. Features that define the type 1 cytokine receptor family include two signature motifs in the extracellular domain and one in the intracellular domain. Four cysteine residues in the extracellular domain are absolutely conserved among all of the type 1 cytokine receptors, and they form two disulfide bridges that are essential for the proper tertiary folding of the ligand-binding domain. A short sequence, which includes a tandem repeat of tryptophan-serine interrupted by a single amino acid (the WSXWS …