RBP4 and obesity due to melanocortin 4 receptor deficiency: Elevated RBP4 levels have been observed in insulin-resistant states such as obesity and type 2 diabetes mellitus (T2DM), implicating it in the development of metabolic syndrome (Klöting et al., 2007; Kovacs et al., 2007; Olsen and Blomhoff, 2020; Flores-Cortez et al., 2022).