SIM1 is a basic helix-loop-helix transcription factor that plays critical roles in hypothalamic and pituitary development. 1 It functions as a DNA-binding transcriptional regulator that forms protein heterodimers to modulate gene expression through RNA polymerase II. 1 Mechanistically, SIM1 is essential for proper development of the paraventricular nucleus (PVN) of the hypothalamus. 1 SIM1 is expressed in hypothalamic neuroendocrine lineages generating oxytocin, arginine vasopressin, and other regulatory hormones. 2 Loss of SIM1 function results in a hypocellular PVN architecture. 1 SIM1 deficiency causes severe monogenic obesity in both mice and humans. 3 SIM1-deficient mice exhibit hyperphagia and severe obesity, 1 while human SIM1 mutations are associated with severe early-onset obesity, hyperphagia, and potentially hypopituitarism affecting multiple hormone axes. 2 Common SIM1 variants show associations with body mass index and weight gain in population studies, particularly in females. 1 Clinically, SIM1 mutations represent an important genetic cause of monogenic obesity. 3 CRISPR-mediated activation of SIM1 rescued obesity in heterozygous mice, suggesting gene therapy potential. 4 Pituitary hormone assessment is recommended in pediatric obesity patients with SIM1 abnormalities. 2