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Your article reports that chemically synthesized erythropoiesis protein (SEP) was more effective than the genetically engineered molecule. Is there speculation on why that is?

Ann Dershowitz
West Orange, N.J.

Chemically constructed SEP molecules are much more consistent in size, shape, and other properties than genetically engineered erythropoietin. The polymer appendages put onto the molecule protect SEP and thus increase its lifetime, which also leads to higher activity .–J. Gorman