Norman Horowitz, 90, the California Institute of Technology biologist who played key roles in understanding a variety of scientific questions, including genetics, evolutionary theory and whether there is life on Mars, died June 1 at his home in Pasadena, Calif. No cause of death was reported.

Dr. Horowitz was best known for conceiving the pyrolytic release experiment, instruments for which were carried to Mars aboard the two Viking landers in 1976.

At the time, researchers thought that potential Martian life would be able to use carbon monoxide or carbon dioxide from the atmosphere, in the absence of water vapor, to grow. The instruments took small samples of Martian soil and incubated them in a light chamber in the presence of the two gases, which had been radioactively labeled.

After 120 hours, any organic compounds that might have formed were broken down under high heat -- a technique called pyrolysis -- and studied with a mass spectrometer to determine whether they had incorporated any radioactive carbon.

They had not, and the experiments were taken as strong proof that life did not then exist on Mars, at least at the two landing sites. Now that researchers have found evidence of water under the Martian surface, however, many believe that the possibility of life still exists.

Dr. Horowitz played an even more important role in pinning down two important concepts in molecular biology. When he began his graduate studies in the late 1930s, very little was known about genes -- what they are made of or how they work.

As a postdoctoral fellow at Stanford University during the World War II years, Dr. Horowitz worked with geneticists George W. Beadle and Edward L. Tatum, who were studying the red mold known as Neurospora crassi, an organism that had not previously been used in genetics research.

That work looked at a series of mutants, each of which required different organic compounds added to its diet. The research led to the creation of the "one gene, one enzyme" concept, which holds that each gene in the bacterial, and human, genome serves as the blueprint for only one enzyme that plays a role in the metabolism of cells.

The work earned Beadle and Tatum half of the 1958 Nobel Prize in physiology/medicine. In his acceptance speech, Beadle credited Dr. Horowitz for much of the work that went into proving the concept.

In a crucial paper in 1945, Dr. Horowitz addressed how complex pathways within cells to synthesize biologically important compounds could have originated.

He pointed out that many such compounds were known to be synthesized step by step using intermediate compounds that apparently had no other roles in the cell. How could such a synthetic pathway have evolved, he asked, if it served no purpose until it was complete?

Dr. Horowitz reasoned that the end product of the pathway was at first obtained by organisms directly from the environment, where it could have been produced by non-biological mechanisms, as has been shown by many researchers.

It was then possible to reasonably assume, he said, that the ability to synthesize the compound biologically could arise through a series of separate single mutations. The first such mutation might, for example, produce the desired end product in one step from some other compound in the environment. The next mutation would produce that product's precursor, and so on until a complete series of reactions would yield the desired product from readily available, simple materials.

Each successive mutation would provide a selective advantage by making the organism less dependent on chemicals in its environment.

"I know of no alternative hypothesis that is equally simple and plausible," Beadle said. Dr. Horowitz's formulation is now widely accepted.

Norman Harold Horowitz was born March 19, 1915, in Pittsburgh. His parents were not interested in science, he said, but they raised three sons who were. The other two were a chemist and a petroleum engineer.

Dr. Horowitz received a degree in zoology from the University of Pittsburgh.

When Beadle left Stanford to become chairman of Caltech's biology division, he brought Dr. Horowitz with him, and the young researcher spent the rest of his career there. He was chairman of the biology division from 1977 until 1980 and retired in 1982.

For five of his working years, he was also on the staff of the Jet Propulsion Laboratory, where he was a key player in the early stages of the search for extraterrestrial life. After his retirement, he wrote a book, "To Utopia and Back: The Search for Life in the Solar System," about that quest and his experiences in the space program.

Dr. Horowitz's wife, the former Pearl Shykin, died in 1985.

Survivors include two children and two grandchildren.

In addition to determining two important concepts in molecular biology, Norman Horowitz led an experiment to see if there was life on Mars.