SIM1 and obesity due to melanocortin 4 receptor deficiency: Mutations in other genes, such as POMC (pro-opiomelanocortin), LEP (leptin), PCSK1 (proprotein convertase subtilisin/kexin type 1), NTRK2 (neurotrophic receptor tyrosine kinase type 2), BDNF (brain-derived neurotrophic factor), and SIM1 (single-minded family bHLH transcription factor 1), are even rarer causes of monogenic obesity [12].