Erythropoietin (Epo) is a primary regulator of erythropoiesis and functions by binding to the erythropoietin receptor (EpoR) on the surface of hematopoietic progenitor cells, followed by receptor-mediated endocytosis of the Epo molecule. The human EpoR (hEpoR) has been known to consist of two molecular weight polypeptides (85 kd and 100 kd) and to exhibit both low and high affinity binding sites for Epo. We have recently cloned and sequenced the gene for the main polypeptide of the hEpoR (Noguchi, et al BLOOD, 78:2548-2556, 1991). The structure of the gene with respect to introns and extrons and possibly transcription regulatory sequences in the DNA surrounding the gene has been elucidated. We are presently studying the tissue and developmental specificity of the gene in transient assays (see Project Number Z0125082-012 LCB) and, as described in this report, in transgenic mice. For these latter studies we have introduced the entire 15 kb cloned fragment of the hEpoR into mouse pronuclei by microinjection and generated several transgenic lines. Hematological parameters were measured in transgenic mice and found within normal limits except for a suggestion of increased reticulocyte levels. Tissue expression was determined in both normal and transgenic mice from several different lines. We found that mouse Epo-R transcripts were detected in bone marrow and spleen, but not in brain, heart, kidney and liver, but that the human Epo-R transcripts were detected in the brain as well as in bone marrow and spleen, but not in heart, kidney and liver. No endogenous mEpoR were detected in brain of transgenic mice. Recently we have found that there is some expression of the endogenous gene very early in the brain but with development this decreases. We are currently studying the biological significance of these observations.