MC4Rs are expressed solely in CNS

Or in other words that melanocortin 4 receptors (MC4Rs) are expressed exclusively in the central nervous system (CNS).

Why is this significant? Because mutations of the MC4Rs in humans are one of the most common (if not the most) single gene mutations causing obesity. And if MC4Rs are only expressed in the CNS, it supports the important role the brain has in the regulation of energy balance, hence the brain pathways are important for obesity. This is opposed to the (now mostly extinct) thinking of obesity as a disease of periphery.

Of note, other peptides expressed exclusively in the brain are POMC (proopriomelanocortin), serotonin 6 receptor, serotonin 2C receptor, agouri related peptide. All of these are also important in the regulation of energy balance.