The Interleukin-5 receptor alpha subunit (IL5Ra) has a single transmembrane domain, a glycosylated extracellular domain and a short (58 amino acids) cytoplasmic tail, containing no tyrosine kinase domain. It binds IL-5 with a relatively low affinity and is not capable of signaling by itself. The alpha subunit has alternatively spliced soluble forms that are capable of binding IL-5 and act as natural antagonists of IL-5 signaling. The cytoplasmic domain of the alpha chain appears to be critical for IL-5 signaling (Takaki et al. 1993). IL5R alpha chain was found to be constitutively associated with JAK2 (Ogata et al. 1998); the same study found that JAK1 was constitutively associated with Bc, though the consensus is that JAK2 is associated with Bc.