LEP and 3-methylglutaconic aciduria type 3: Since marrow adipocytes themselves produce high levels of leptin (32) and direct infusion of leptin into the tibial marrow cavity increases osteoblast surface and reduces marrow adipogenesis (33), the presence of weakened limb bones in the context of a sustained increase in marrow adiposity in Costeff syndrome mice implies that this action of leptin is dependent upon mitochondrial activity.