Mild mutations in the PCSK1 gene (proprotein convertase subtilisin/kexin type 1) are associated with obesity [10], and more severe, rare mutations have increasingly been recognized as a cause of malabsorptive diarrhea and other endocrinopathies in a disorder called proprotein convertase 1/3 (PC1/3) deficiency (OMIM: 600955) [11]. Here, PCSK1 is linked to obesity due to melanocortin 4 receptor deficiency.