EXT1 and obesity due to melanocortin 4 receptor deficiency: Interestingly, this axis was decreased during obesity, and myeloid cell-specific deletion of the heparan sulfate synthetic enzyme (Ext1) impaired mitochondrial uptake by macrophages, leading to dysregulated energy homeostasis and a susceptibility to diet-induced obesity in Lyz2cre-Ext1flox/flox mice (Brestoff et al., 2021; Fig. 2 C).