The GM-CSF receptor is expressed primarily on myeloid cells and consists of two separate subunits, both of which belong to the type I cytokine receptor family. The a subunit binds GM-CSF with high specificity but low affinity while the 0chain is shared with the IL-3 and IL-5 receptors and converts each of these low-affinity interactions into one of high affinity. The a chain has only a short cytoplasmic tail which is nevertheless essential for intracellular signaling but the longer cytoplasmic tail of the 0 chain appears to transmit most of the biological signals. Known signaling pathways include activation of the JAK2/STAT5 and MAP kinase pathways and induction of cell survival, proliferation, and functional activation.