Loss-of-function (LOF) mutations in the IGSF1 gene have been described in the X-linked IGSF1 deficiency syndrome [8,9], a rare genetic condition characterized by central hypothyroidism, macroorchidism, variable prolactin and growth hormone levels, delayed puberty, obesity, and infertility, which can manifest in a range of clinical phenotypes [10]. The gene discussed is IGSF1; the disease is obesity due to melanocortin 4 receptor deficiency.