Subnormal vasopressin and thirst response to rising osmotic stimuli, with normal osmoregulatory point. Hypothesized to be a result of partial damage of the osmoreceptors. Described in patients with microcephaly and dysplastic corpora callosa. Baroreceptor-regulated vasopressin release is often intact, as is the release in response to insulin-provoked hypoglycemia and emesis. Appears to be increased renal sensitivity to vasopressin, and patients may retain capability of limiting free water excretion by concentrating the urine. This evidence concerns the gene INS and microcephaly.