It will thus be interesting to evaluate the FGF21 gene network for mutations in patients with functional hypogonadotropic hypogonadism (i.e., hypothalamic amenorrhea (Gordon, 2010) and obesity‐related hypogonadotropic hypogonadism; Giagulli et al, 1994), as environmental cues related to energy balance are critical for reproductive fitness. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.