APOE and obesity due to melanocortin 4 receptor deficiency: The mechanism for the association of APOE with obesity and body fat mass is speculated to be attributed to its role in regulating the expandability and functionality of adipose tissues (Huang et al., 2006; Arbones-Mainar et al., 2008; Huang et al., 2009a; Huang et al., 2009b; Li and Liu, 2014).