PEX11A and obesity due to melanocortin 4 receptor deficiency: FAS catalyses the first committed step in de novo lipogenesis, which has been implicated in obesity and insulin resistance.26, 27, 28 As shown in Figure 7C, on a chow diet, FAS mRNA levels in BAT of Pex11a−/− mice were significantly higher than those in BAT of WT mice by approximately 10‐fold.