EPO via Total Synthesis Erythropoietin (EPO) is a hormone involved in the production of red blood cells. Synthetic EPO produced via genetically engineered cell cultures is used to treat anemia and—more controversially—to boost athletic performance. EPO is a glycoprotein, and though its protein component is well-defined, both natural and synthetic EPO exhibit a wide range of attached oligosaccharides. Wang et al. (p. 1357 ; see the Perspective by Hsieh-Wilson and Griffin ) prepared an EPO sample by a chemical synthesis that maintains a uniform pattern of attached sugars throughout, which may prove helpful in the analysis of how variation in the sugar components of EPO impact function.