A study by Krogstad et al. analyzed the activity of major CYP450 isoforms in the liver and small intestine samples from patients with obesity: CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A. This evidence concerns the gene CYP2D6 and obesity due to melanocortin 4 receptor deficiency.