: The cytokine receptor superfamily is a group of transmembrane proteins, characterized by a common extracellular structure-two barrels composed of seven [beta] strands each. Over the past year, several new members of the cytokine receptor family have been described, and new insights have been made into how related receptors share common subunits. Also this year, the ligand for the orphan cytokine receptor c-Mpl was identified and shown to stimulate megakaryocyte development and thrombopoiesis. Progress has been made in unraveling the precise atomic basis for ligand-receptor interactions and the role that subunit association plays in receptor activation. Finally, advances have been made in understanding the organization of the intracellular domain and how signaling to the nucleus is achieved. Together, these new results have led to a greater appreciation of the role that cytokine receptors play in the regulation of proliferation and lineage-specific differentiation during hematopoiesis.