Interleukin 5 (IL-5) is a soluble T cell-derived factor, also known as T cellreplacing factor (TRF), that induces B cell and eosinophil growth and differentiation. IL-5 exerts its biological activity through the IL-5 receptor (IL-5R), which is composed of two chains: an (alpha) chain that binds IL-5 with low affinity and a (beta) chain that does not bind IL-5, but together with the IL-5 (alpha) chain, constitutes the high affinity IL-5 receptor. The cytoplasmic domain of both the (alpha) and (beta) chains is essential for signal transduction. Specifically, the membrane-proximal proline-rich sequence of the cytoplasmic domain of the IL-5R receptor (alpha) subunit, IL-5R(alpha), is critical for the IL-5 induced proliferative response, expression of nuclear proto-oncogenes and tyrosine phosphorylation of cellular proteins, such as JAK1 and JAK2.