In order to further characterize mitochondrial function in these individuals we measured insulin-stimulated rates of muscle mitochondrial ATP synthesis and changes in intramyocellular inorganic phosphate (Pi) concentrations using 31P MRS in a similar group of young, lean, IR offspring of parents with type 2 diabetes and body mass index (BMI)–age–activity-matched control participants. The gene discussed is INS; the disease is type 2 diabetes mellitus.