To figure out the role of the seven SIRT genes, both Guarente and Sinclair engineered two sets of genetically altered mice. For each SIRT gene, one strain lacks the gene entirely and another makes extra amounts of the gene’s product. The knockout mice, by their deficiencies, should show what the lost gene does; its effects will be larger in the overexpressor mice.

The SIRTs intervene in the body’s metabolism in intricate ways that are only beginning to be understood. Mice that overexpress SIRT1 show eight properties of caloric restriction, including low cholesterol and low glucose and insulin blood levels, Dr. Guarente said in a recent talk at the Mount Sinai School of Medicine.

NICHOLAS WADE