PPARδ is the dominant isoform in adult skeletal muscle, and mice with skeletal muscle–restricted PPARδ deficiency share many of the same characteristics as mice with skeletal muscle–specific KLF15 deficiency: both strains suffer from abnormalities in exercise capacity, lipid utilization, and oxidative metabolism and are susceptible to diet-induced obesity (25, 26, 27). Here, KLF15 is linked to obesity due to melanocortin 4 receptor deficiency.