While rodents rely to a large extent on oestrogens formed from androgens for their masculinisation, in humans signalling downstream of the androgen receptor appears to be sufficient for masculinisation of anatomy, as evidenced by rare cases of 5a reductase deficiency (which prevents the synthesis of the potent androgen receptor ligand DHT), congenital complete androgen insensitivity (where XY individuals have external genitalia that are phenotypically female) and even more rare forms of aromatase deficiency (where XY individuals are phenotypically male) [54, 55]. The gene discussed is AR; the disease is hyperinsulinemic hypoglycemia, familial, 4.