PCSK1 and obesity due to melanocortin 4 receptor deficiency: Individuals with inactivating mutations in the PCSK1 gene have several phenotypic similarities with PWS patients, which raises the possibility of a role of PC1/3 (the gene product of PCSK1) activity in several clinical aspects of PWS patients, including severe early-onset obesity, hypogonadotropic hypogonadism, and growth retardation [15].