XCR1 and obesity due to melanocortin 4 receptor deficiency: Recently, it has been discovered that bile acid receptors, such as farnesoid X receptor (FXR) and G protein-coupled receptor 5 (TGR5) (Kuipers et al., 2004; Deutschmann et al., 2018), play a role in regulating obesity, diabetes, and nonalcoholic fatty liver disease by activating transcription factors related to lipid, cholesterol, and carbohydrate metabolism (Chávez-Talavera et al., 2017).