Norman Harold Horowitz (March 19, 1915 – June 1, 2005) was an American geneticist at Caltech who achieved national fame as the scientist who devised experiments to determine whether life might exist on Mars.

In 1965 he began work with the Jet Propulsion Laboratory in Pasadena, serving for five years as chief of JPL's bioscience section and as a member of the science teams for the Mariner and Viking missions to Mars.

Horowitz earned his BS in biology at the University of Pittsburgh in 1936 where his experience conducting research as an undergraduate helped to persuade him to pursue further graduate training in science.

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.

His discovery helped to clinch the case for George Beadle and Edward Tatum's "one gene-one enzyme hypothesis" (a term Horowitz coined for their concept).

Metzenberg[2] remarked on the stubborn courage required for Horowitz to espouse and defend the initially unpopular idea of “one gene-one enzyme” and to see it through to general acceptance.

[8] Dr Beadle (1958 Nobel Prize winner in Medicine) gave credit to Dr. Horowitz for his original work on biological reactions.