RBP4 and obesity due to melanocortin 4 receptor deficiency: Retinol binding protein 4 (RBP4), a newly discovered adipose factor, is the main transporter of retinol and is related to the dysregulation of energy metabolism, insulin resistance, diabetes mellitus, and obesity [13–15]. Most recent studies have shown that RBP4 is permeable to the blood-brain barrier (BBB) and increases with the dysfunction of BBB [16].