IL7 is secreted constitutively into the conditioned medium of adherent bone marrow stromal cells and thymic cells. Mouse and human keratinocytes have been shown also to express and secrete IL7.Human (152 amino acids; 17.4 kDa) and murine IL7 (129 amino acids) show 60 % sequence homology at the protein level. The human IL7 receptor is an integral strongly glycosylated membrane proteins of 76 kDa expressed on activated T-cells. This receptor has been designated as CD127. IL7 receptors are expressed on pre-B-cells and their progenitors. They are not expressed on mature B-cells. IL7 receptors are expressed also on bone marrow macrophages. Functional IL7 receptors are found on the cell surface of multiphenotypic, biphenotypic, and immature lymphoid progenitors of B- cells with the gene arrangement of the heavy immunoglobulin chain such as those observed in the germ line