For additional details see Ebert et al, Identification and Small Molecule Inhibition of an ATF4-dependent Pathway to Age-related Skeletal Muscle Weakness and Atrophy. Overall design: Quadriceps femoris muscles were harvested from 22-month-old muscle-specfic ATF4 knockout (ATF4 mKO) mice and littermate controls. mRNA levels in ATF4 mKO muscles were normalized to levels in littermate control muscles.