PCSK1 and obesity due to melanocortin 4 receptor deficiency: Both rare and common mutations in PCSK1 (also known as prohormone convertase 1/3 or PC1/3) have been associated with obesity [2], and in various human genetic studies with a wide spectrum of metabolic phenotypes, including hyperphagia, intestinal malabsorption, gastrointestinal complications, diabetes insipidus, reactive hypoglycemia, and others [3].