 He served as chair of the Biology Division from 1977 to 1980, and became professor emeritus in 1982.As a scientist, Horowitz is best known for his discovery and demonstration in 1944 that a metabolic pathway is a series of steps, each catalyzed by a single enzyme.Working with "Neurospora crassa", Horowitz demonstrated that each step in the metabolism of arginine from its precursors depends on the intactness of a single gene.