BDNF and obesity due to melanocortin 4 receptor deficiency: In addition, the obesity phenotype in patients with 6q16 deletion is likely explained by haploinsufficiency of the transcription factor single-minded 1 (SIM1) gene [6], whereas obesity susceptibility in WAGR syndrome mainly depends on haploinsufficiency for the brain-derived neurotrophic factor (BDNF) gene [7].