PCSK1 and obesity due to melanocortin 4 receptor deficiency: Complete prohormone convertase 1 deficiency results in early on-set severe obesity, hyperphagia, hypoglycemia, and endocrine dysfunction (Farooqi et al., 2007a; Farooqi et al., 2007b; Farooqi et al., 2007c; Jackson et al., 2003; Jackson et al., 1997).