Acquired obesity in twins was associated with a significant down-regulation of the genes encoding the rate-limiting enzymes in fatty acid synthesis and β-oxidation (Table S2), and especially interesting was the down-regulation of mitochondrial acetyl-coenzyme A carboxylase beta (ACACB), an important regulator of fatty acid oxidation. The gene discussed is ACACB; the disease is obesity due to melanocortin 4 receptor deficiency.