IRS1 and Insulin resistance: Our aim was to study both liver and skeletal muscle mitochondrial function in a well-established mouse model of insulin resistance, IR+/−-IRS-1+/− double heterozygous (IR-IRS1dh) mice [9], at advanced age, since moderately reduced insulin signaling was implicated in increased lifespan [10] and these mice have been rarely analyzed at older age [9,11,12].