The biological effects of IL-7 are mediated by the binding of IL-7 to the specific cell surface receptor complex. The functional high-affinity IL-7 receptor (IL-7 R) comprises the IL-7 R alpha chain in association with the common gamma chain (gamma c). Both IL-7 R alpha and gamma c are members of the hematopoietin receptor superfamily. Human and mouse IL-7 R alpha show 64% amino acid sequence identity. IL-7 R alpha transcripts were observed in murine spleen, thymus, fetal liver, developing T cells, B cells, mature T cells and bone marrow-derived macrophages.