Erythropoietin (Epo), a hormone made by the kidney and secreted into the plasma, is the master regulator of erythropoiesis (red blood cell [RBC] production). Adequate Epo levels are essential for maintaining normal RBC mass. Elevated plasma Epo concentrations lead to increased RBC production (polycythemia) resulting in an increased risk of thrombosis, while low plasma Epo levels result in decreased RBC production (anemia). Substantial variations in plasma Epo levels have been documented in healthy individuals; for example, the normal Epo level in one major reference lab is 2.6-18.5 IU/L; a seven fold difference between the lowest and highest limits of normal. As a complex genetic trait, this variation in Epo levels are determined by both environmental and genetic factors. In this proposal, we aim to identify the common genetic determinants of plasma Epo levels using two previously collected cohorts of healthy young students and siblings. Plasma from these participants has been collected and stored, and genome wide single nucleotide polymorphism genotyping has been performed. In this proposal, we will first measure plasma Epo concentrations in these samples (N = 3700) using a high throughput immunoassay. We will examine the immunoassay results using cutting edge techniques to identify batch effects causing variation in Epo levels not attributable to biologic differences in samples. Then, in order to determine the fraction of variation in Epo levels that are attributable to genetic factors, we will precisely estimate the heritability of Epo levels using the sibling structure of the cohorts and genotyping methods. We will next perform genome-wide association studies to identify common genetic variants associated with serum Epo levels. We predict that a strong signal will be detected with variants in the locus encoding Epo, a cis quantitative trait loci, but also anticipate the statistical power to detect signals at other loci in the genome which suggest a biologic interaction with Epo. The use of innovative methods to detect and correct batch effects and the use of samples from young, healthy participants will maximize the power to detect genetic associations. Findings from this project will form the basis for larger-scale proposals with extramural funding to investigate the mechanism by which the uncovered associations result in altered Epo levels. Ultimately, these findings may identify novel therapeutic targets for the treatment of anemia and/or polycythemia.