Studies have shown that at least 80 SLC proteins are associated with human metabolic diseases, including obesity, T2D, NAFLD, and multiple insulin resistance-related metabolic diseases (Lin et al., 2015a; Schumann et al., 2020), emphasizing the importance of SLC family in regulating normal physiological and pathological functions. Here, CCL21 is linked to obesity due to melanocortin 4 receptor deficiency.