AR and obesity due to melanocortin 4 receptor deficiency: The blood lipids (e.g., free fatty acid and triglyceride) and cytokines (e.g., leptin) in mice with global AR knockout (ARKO) or tissue-specific knockout (e.g., liver and skeletal muscle) are significantly increased, and they are prone to suffer from obesity, insulin resistance (IR), and liver fibrosis (7, 8).