IL5RA（INTERLEUKIN 5 RECEPTOR, ALPHA），also known as CD125，is a subunit of the Interleukin-5 receptor. IL5RA also denotes its human gene. The protein encoded by this gene is an interleukin 5 specific subunit of a heterodimeric cytokine receptor which is composed of a ligand specific alpha subunit and a signal transducing beta subunit shared by the receptors for interleukin 3 (IL3), colony stimulating factor 2 (CSF2/GM-CSF), and interleukin 5 (IL5). The IL5RA gene is mapped to 3p26-p24 by Southern blot analysis of DNA from a panel of mouse-human hybrid somatic cell lines complemented by in situ hybridization. And its Cytogenetic location is 3p26.2. Within this 15-residue stretch of IL5RA, the C-terminal phenylalanine is critical. The binding of this protein to IL5 depends on the beta subunit which is activated by the ligand binding and is required for the biological activities of IL5.