NTRK2 and obesity due to melanocortin 4 receptor deficiency: Mutations in the leptin gene (LEP), leptin receptor (LEPR), pro-opiomelanocortin (POMC), proconvertase 1 (PC1), melanocortin receptor 4 (MC4R), melanocortin receptor 3 (MC3R), neurotrophic tyrosine kinase receptor type 2 (NTRK2) and many others have been recognized as promoters of morbid obesity and rare forms of human obesity [72].